BUG Fixing incorrect error message on failed UploadField upload

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.
Merge pull request #3341 from halkyon/summary_error_on_forbidden

BUG Fixing incorrect error message on failed UploadField upload