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