Make it easier to use print_r() debugging

Summary:
The fixed-position side nav background thing tends to make looking at print_r() output hard. Also, it breaks Ajax, etc.

  - Loudly call out unexpected output on normal pages, to catch extra spaces before `<?php`, etc.
  - Display unexpected output in an attractive panel on normal pages.
  - Log unexpected output instead of breaking Ajax.

Test Plan:
{F32267}

Also triggered various fatals and verified they still show the right messages (no blank pages).

Reviewers: vrana, btrahan, chad

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4892