Treat PHP7 "Throwable" exceptions like other unhandled "Exception" cases in the worker queue

Summary: See PHI1745. Under PHP7, errors raised as Throwable miss this "generic exception" logic and don't increment their failure count. Instead, treat any "Throwable" we don't recognize like any "Exception" we don't recognize.

Test Plan:

• Under PHP7, caused a worker task to raise a Throwable (e.g., call to undefined method, see D21270).
• Ran bin/worker execute --id ....
• Before: worker failed, but did not increment failure count.
• After: worker fails and increments failure count as it would for other types of unknown error.

Differential Revision: https://secure.phabricator.com/D21271