R1:6f388f97d959

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()…