Give users an explicit error if they try to upload a too-large diff to Differential

Summary:
Ref T8612. If a change affects more than 10K paths + hunks, tell the user it's too big and don't bother trying to write it. We're mostly bounded by INSERTs here.

Also, fix an issue with file upload errors. The keys are real PHP constants, but were accidentally converted to strings in D12797, causing every error to show as "unknown error".

Test Plan:

Reviewers: joshuaspence

Reviewed By: joshuaspence

Maniphest Tasks: T8612

Differential Revision: https://secure.phabricator.com/D14977