Add a lint check for catching "Exception" without catching "Throwable"

Summary:
Ref T13588. For consistency of behavior between versions on either side of PHP7, any "catch (Exception)" block should generally have a "catch (Throwable)" block in the same catch list.

Raise a lint warning when "Exception" is caught without also catching "Throwable", since this is almost certainly not desired.

Test Plan: Added tests.

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21542