If applying custom Upload_Validator rules, uploads will fail POST with
a 403 to UploadField/upload as the upload was rejected, but UploadField
still shows that the upload was successful.
jqXHRorError returns either an error string, e.g. "Forbidden" or an
object. If it's a string, ensure that the upload summary shows an
error message. If it's an object and has a status of 200, show the
success message.