R1:f1a36cf3c8e0

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).