(stable) Let "phlog()" log Throwables

Summary:
Ref T13250. If you "phlog()" a Throwable, it doesn't recognize that it's an exception and sends you down the "unknown type of object" pathway. You get a one-line "Object Error" message in the log.

Instead, treat it more like an Exception and log a message + stack trace.

Test Plan: Logged some Throwables, got sensible information about them.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13250

Differential Revision: https://secure.phabricator.com/D20143