Allow libphutil to start up in the presence of missing global functions

Summary:
See D3393.

• When we fail to load a symbol, try to load everything else before we throw. Throw an arbitrary one of the failure(s) since it's not terribly important which one we pick.
• When loading global functions fails, proceed anyway.

Test Plan: Removed a function, ran arc liberate, it was nuked from the library map.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3395