Pass all Throwables to Exception Handlers, not just Exceptions

Summary:
Ref T12855. PHP7 introduced "Throwables", which are sort of like super exceptions. Some errors that PHP raises at runtime have become Throwables instead of old-school errors now.

The major effect this has is blank pages during development under PHP7 for certain classes of errors: they skip all the nice "show a pretty error" handlers and

This isn't a compelete fix, but catches the most common classes of unexpected Throwable and sends them through the normal machinery. Principally, it shows a nice stack trace again instead of a blank page for a larger class of typos and minor mistakes.

Test Plan:
Before: blank page. After:

Reviewers: chad, amckinley

Reviewed By: chad

Maniphest Tasks: T12855

Differential Revision: https://secure.phabricator.com/D18136