Catch unhandled exceptions in index.php

Summary:
When there is an exception in index.php then we currently get only blank screen.
Print it instead.

Test Plan: Thrown exceptions on several places of index.php and controller, got best results.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3619