If no file is specified when restoring a course backup then Moodle displays an error

When the user tries to restore a course and forgets to upload a backup file to the Files form element, Moodle shows the error "error/invalidrestorefile". The handling of this is inelegant and should be elegant.

To recreate:

1. In any course navigate to Course administration > Restore.
2. Without uploading a backup file click the Restore button.

Expected result:

• The fact that there is no backup file is handled elegantly by Moodle.

Actual result:

• An error is shown with no help text.