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

api4/system_test.go

@@ -478,7 +478,7 @@ func TestS3TestConnection(t *testing.T) {
 		config.FileSettings.AmazonS3Bucket = model.NewString("Wrong_bucket")
 		_, resp = th.SystemAdminClient.TestS3Connection(&config)
 		CheckInternalErrorStatus(t, resp)
-		assert.Equal(t, "Unable to create bucket.", resp.Error.Message)
+		assert.Equal(t, "api.file.test_connection.app_error", resp.Error.Id)
 
 		*config.FileSettings.AmazonS3Bucket = "shouldcreatenewbucket"
 		_, resp = th.SystemAdminClient.TestS3Connection(&config)