R1:bdecadfd119e

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.

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