Enforce upload size limits and transport exceptions with appropriate response encoding

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