Returning simple text error message instead of HTML for requests where HTML won't be rendered for the user. Fixes T2401.

Summary: If the check is too much, let me know. I noticed you send over __ajax__=true, so I figured it was safest to evaluate existance and value.

Test Plan: Included unit test. Would have included a test where __ajax__ and __conduit__ are not set, but without mocking this gives an uncatchable Fatal Error. If you want me to include it, just direct me on the mocking strategy.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2401

Differential Revision: https://secure.phabricator.com/D4719