Avoid throwing in ApacheThriftGen.__init__

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/43046341

Bugs closed: 842

Reviewed at https://rbcommons.com/s/twitter/r/1428/
Sync of https://github.com/pantsbuild/pants/commit/536e5e92b6bdaafeb76d0d23541c82900b036b6f