Provide more detailed exceptions when symbols fail to load

Summary: Give the user everything we can when this happens.

Test Plan: Added a new ZZ(); call to trigger the first case. Renamed Future to Futurexyz to trigger the second case. Grepped for other throwers.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D5037