Remove usages of AppError on filesstore service (#15841)

* Remove usages of AppError on filesstore service

* Fixing a golint error

* Fixing shadowed variable

* Adding err.Error() to the NewAppError calls

* Fixing tests

* Adding missed translations

* Fix error handling and updating the translation that affects it

* Fixing two typos

i18n/en.json

@@ -1317,39 +1317,23 @@
     "translation": "Unable to create the emoji. An error occurred when trying to open the attached image."
   },
   {
-    "id": "api.file.append_file.no_exist.app_error",
-    "translation": "File does not exist."
-  },
-  {
-    "id": "api.file.append_file.opening.app_error",
-    "translation": "Encountered an error opening the file."
-  },
-  {
-    "id": "api.file.append_file.s3.app_error",
-    "translation": "Encountered an error appending to a file in S3."
-  },
-  {
-    "id": "api.file.append_file.writing.app_error",
-    "translation": "Encountered an error writing to S3."
+    "id": "api.file.append_file.app_error",
+    "translation": "Unable to append data to the file."
   },
   {
     "id": "api.file.attachments.disabled.app_error",
     "translation": "File attachments have been disabled on this server."
   },
   {
-    "id": "api.file.file_exists.exists_local.app_error",
+    "id": "api.file.file_exists.app_error",
     "translation": "Unable to check if the file exists."
   },
   {
-    "id": "api.file.file_exists.s3.app_error",
-    "translation": "Unable to check if the file exists."
+    "id": "api.file.file_reader.app_error",
+    "translation": "Unable to get a file reader."
   },
   {
-    "id": "api.file.file_size.local.app_error",
-    "translation": "Unable to get the file size."
-  },
-  {
-    "id": "api.file.file_size.s3.app_error",
+    "id": "api.file.file_size.app_error",
     "translation": "Unable to get the file size."
   },
   {
@@ -1373,56 +1357,36 @@
     "translation": "Unable to get public link for file. File must be attached to a post that can be read by the current user."
   },
   {
-    "id": "api.file.move_file.copy_within_s3.app_error",
-    "translation": "Unable to copy file within S3."
+    "id": "api.file.list_directory.app_error",
+    "translation": "Unable to list directory."
   },
   {
-    "id": "api.file.move_file.delete_from_s3.app_error",
-    "translation": "Unable to delete file from S3."
-  },
-  {
-    "id": "api.file.move_file.rename.app_error",
-    "translation": "Unable to move file locally."
-  },
-  {
-    "id": "api.file.new_backend.s3.app_error",
-    "translation": "Encountered an error opening a connection to S3."
+    "id": "api.file.move_file.app_error",
+    "translation": "Unable to move file."
   },
   {
     "id": "api.file.no_driver.app_error",
     "translation": "No file driver selected."
   },
+  {
+    "id": "api.file.read_file.app_error",
+    "translation": "Unable to read the file."
+  },
   {
     "id": "api.file.read_file.reading_local.app_error",
     "translation": "Encountered an error reading from local server file storage."
   },
   {
-    "id": "api.file.read_file.s3.app_error",
-    "translation": "Encountered an error reading from S3 storage."
+    "id": "api.file.remove_directory.app_error",
+    "translation": "Unable to remove the directory."
   },
   {
-    "id": "api.file.reader.reading_local.app_error",
-    "translation": "Encountered an error opening a reader from local server file storage."
+    "id": "api.file.remove_file.app_error",
+    "translation": "Unable to remove the file."
   },
   {
-    "id": "api.file.reader.s3.app_error",
-    "translation": "Encountered an error opening a reader from S3 storage."
-  },
-  {
-    "id": "api.file.test_connection.local.connection.app_error",
-    "translation": "Don't have permissions to write to local path specified or other error."
-  },
-  {
-    "id": "api.file.test_connection.s3.bucket_create.app_error",
-    "translation": "Unable to create bucket."
-  },
-  {
-    "id": "api.file.test_connection.s3.bucket_exists.app_error",
-    "translation": "Error checking if bucket exists."
-  },
-  {
-    "id": "api.file.test_connection.s3.list_objects.app_error",
-    "translation": "Error trying to list objects."
+    "id": "api.file.test_connection.app_error",
+    "translation": "Unable to access the file storage."
   },
   {
     "id": "api.file.upload_file.incorrect_channelId.app_error",
@@ -1465,16 +1429,8 @@
     "translation": "Unable to upload file {{.Filename}}. {{.Length}} bytes exceeds the maximum allowed {{.Limit}} bytes."
   },
   {
-    "id": "api.file.write_file.s3.app_error",
-    "translation": "Encountered an error writing to S3."
-  },
-  {
-    "id": "api.file.write_file_locally.create_dir.app_error",
-    "translation": "Encountered an error creating the directory for the new file."
-  },
-  {
-    "id": "api.file.write_file_locally.writing.app_error",
-    "translation": "Encountered an error writing to local server storage."
+    "id": "api.file.write_file.app_error",
+    "translation": "Unable to write the file."
   },
   {
     "id": "api.image.get.app_error",
@@ -6026,6 +5982,10 @@
     "id": "ent.actiance.export.marshalToXml.appError",
     "translation": "Unable to convert export to XML."
   },
+  {
+    "id": "ent.actiance.export.write_file.appError",
+    "translation": "Unable to write the export file."
+  },
   {
     "id": "ent.api.post.send_notifications_and_forget.push_image_only",
     "translation": " attached a file."
@@ -6106,6 +6066,10 @@
     "id": "ent.compliance.csv.warning.appError",
     "translation": "Unable to create the warning file."
   },
+  {
+    "id": "ent.compliance.csv.write_file.appError",
+    "translation": "Unable to write the csv file."
+  },
   {
     "id": "ent.compliance.csv.zip.creation.appError",
     "translation": "Unable to create the zip export file."
@@ -6122,6 +6086,10 @@
     "id": "ent.compliance.global_relay.rewind_temporary_file.appError",
     "translation": "Unable to re-read the Global Relay temporary export file."
   },
+  {
+    "id": "ent.compliance.global_relay.write_file.appError",
+    "translation": "Unable to write the global relay file."
+  },
   {
     "id": "ent.compliance.licence_disable.app_error",
     "translation": "Compliance functionality disabled by current license. Please contact your system administrator about upgrading your enterprise license."
@@ -8246,30 +8214,6 @@
     "id": "system.message.name",
     "translation": "System"
   },
-  {
-    "id": "utils.file.list_directory.local.app_error",
-    "translation": "Encountered an error listing directory from local server file storage."
-  },
-  {
-    "id": "utils.file.list_directory.s3.app_error",
-    "translation": "Encountered an error listing directory from S3."
-  },
-  {
-    "id": "utils.file.remove_directory.local.app_error",
-    "translation": "Encountered an error removing directory from local server file storage."
-  },
-  {
-    "id": "utils.file.remove_directory.s3.app_error",
-    "translation": "Encountered an error removing directory from S3."
-  },
-  {
-    "id": "utils.file.remove_file.local.app_error",
-    "translation": "Encountered an error removing file from local server file storage."
-  },
-  {
-    "id": "utils.file.remove_file.s3.app_error",
-    "translation": "Encountered an error removing file from S3."
-  },
   {
     "id": "utils.mail.connect_smtp.helo.app_error",
     "translation": "Failed to set HELO."