Make libphutil scripts exit with an error code when terminated by an uncaught exception

Summary:
We install an exception handler but PHP gets all awkward when we do. See
comment. This restores sensible behavior and sanity to the world.

Test Plan:
Ran "phd debug IRC derpderp" and had the bot exit with an error instead of
exiting "successfully".

Reviewed By: codeblock
Reviewers: codeblock
CC: aran, codeblock
Differential Revision: 535