R1:7b5f47b17d6e

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…