Improve Phabricator behavior for fatal errors

Summary:

• Exceptions on the rendering pathway currently go uncaught and result in a

blank page. Commonly, this is a bad require_celerity_resource() call. Although
we can't safely render a page if the rendering pathway is broken, we can show a
useful message.

• When PHP exits because of a fatal error, there is an opportunity to run code

in the shutdown handler. This allows us to show messages at least some of the
time, e.g. "call to unknown function derp() in somefile.php at line 99"

• flip dem tables

Test Plan: Added fatals ("derp();") and rendering exceptions
("require_celerity_resource('does-not-exist')") to a controller and verified
that the error handling behavior is now more useful.
Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, epriestley
Differential Revision: 680