Summary: - When a user uploads an oversized file, throw an exception. - When an uncaught exception occurs during a Conduit request, return a Conduit response. - When an uncaught exception occurs during a non-workflow Ajax request, return an Ajax response. Test Plan: - Uploaded overlarge files. - Hit an exception page with ?__ajax__=1 and ?__conduit__=1 Reviewers: btrahan, vrana, jungejason Reviewed By: btrahan CC: aran Maniphest Tasks: T875, T788 Differential Revision: https://secure.phabricator.com/D2385