Fixed error message displayed for FileWillBeUploaded rejections (#10535)

#### Summary
Fixed the error message displayed when an uploading file gets rejected by a plugin's FileWillBeUploaded hook.

#### Ticket Link
https://mattermost.atlassian.net/browse/MM-14803

app/file.go

@@ -648,8 +648,8 @@ func (t *uploadFileTask) runPlugins() *model.AppError {
 		replacementInfo, rejectionReason := hooks.FileWillBeUploaded(pluginContext,
 			t.fileinfo, t.newReader(), buf)
 		if rejectionReason != "" {
-			rejectionError = t.newAppError("api.file.upload_file.read_request.app_error",
-				rejectionReason, http.StatusBadRequest)
+			rejectionError = t.newAppError("api.file.upload_file.rejected_by_plugin.app_error",
+				rejectionReason, http.StatusForbidden, "Reason", rejectionReason)
 			return false
 		}
 		if replacementInfo != nil {

i18n/en.json

@@ -1252,6 +1252,10 @@
     "id": "api.file.upload_file.too_large_detailed.app_error",
     "translation": "Unable to upload file {{.Filename}}. {{.Length}} bytes exceeds the maximum allowed {{.Limit}} bytes."
   },
+  {
+    "id": "api.file.upload_file.rejected_by_plugin.app_error",
+    "translation": "Unable to upload file {{.Filename}}. Rejected by plugin: {{.Reason}}"
+  },
   {
     "id": "api.file.write_file.s3.app_error",
     "translation": "Encountered an error writing to S3"