Add PhutilProxyException to permit nested exceptions prior to PHP 5.3

Summary:

• Prior to PHP 5.3, exceptions aren't nestable.
• Fake it.

Test Plan: Ran unit tests. Threw a proxy exception, verified sensible console output.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2193

Differential Revision: https://secure.phabricator.com/D4208