Migrating AppError to error for mailservice (#16810)

* Migrating AppError to error for mailservice

* Updating i18n strings

* Fixing shadow variable problem

* Addressing PR review comments

* fixing test

i18n/en.json

@@ -6670,6 +6670,10 @@
     "id": "ent.message_export.global_relay_export.deliver.to_address.app_error",
     "translation": "Unable to set the email To address."
   },
+  {
+    "id": "ent.message_export.global_relay_export.deliver.unable_to_connect_smtp_server.app_error",
+    "translation": "Unable to connect to the smtp server"
+  },
   {
     "id": "ent.message_export.global_relay_export.deliver.unable_to_get_file_info.app_error",
     "translation": "Unable to get the information of the export temporary file."
@@ -8394,46 +8398,6 @@
     "id": "system.message.name",
     "translation": "System"
   },
-  {
-    "id": "utils.mail.connect_smtp.helo.app_error",
-    "translation": "Failed to set HELO."
-  },
-  {
-    "id": "utils.mail.connect_smtp.open.app_error",
-    "translation": "Failed to open connection."
-  },
-  {
-    "id": "utils.mail.connect_smtp.open_tls.app_error",
-    "translation": "Failed to open TLS connection."
-  },
-  {
-    "id": "utils.mail.new_client.auth.app_error",
-    "translation": "Failed to authenticate on SMTP server."
-  },
-  {
-    "id": "utils.mail.sendMail.attachments.write_error",
-    "translation": "Failed to write attachment to email"
-  },
-  {
-    "id": "utils.mail.send_mail.close.app_error",
-    "translation": "Failed to close connection to SMTP server."
-  },
-  {
-    "id": "utils.mail.send_mail.from_address.app_error",
-    "translation": "Error setting \"From Address\""
-  },
-  {
-    "id": "utils.mail.send_mail.msg.app_error",
-    "translation": "Failed to write email message."
-  },
-  {
-    "id": "utils.mail.send_mail.msg_data.app_error",
-    "translation": "Failed to add email message data."
-  },
-  {
-    "id": "utils.mail.send_mail.to_address.app_error",
-    "translation": "Error setting \"To Address\"."
-  },
   {
     "id": "web.command_webhook.command.app_error",
     "translation": "Couldn't find the command."