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