show the current size limit when a file upload fails

Summary: Show the php.ini setting for max upload or alterantively the in phabricator configured one. Fixes T6663

Test Plan: changed php.ini and alternatively phabricator file upload size settings to minimal values and try to upload a larger file

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, btrahan, chad, epriestley

Maniphest Tasks: T6663

Differential Revision: https://secure.phabricator.com/D11031