Return HTTP 500, not HTTP 200, on exception pages
Summary: Ref T4064. The response code here isn't normally relevant, but we can hit these via git clone http://../, etc., and it's clearly more correct to use HTTP 500.
Test Plan: Added a fake throw new Exception() and verified I got an HTTP 500 response.
Reviewers: jamesr, btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T4064
Differential Revision: https://secure.phabricator.com/D7507