Recover more cleanly from worker tasks with unconstructable classes

Summary:
This is unusual, but if getWorkerInstance() throws we end up with an undefined $worker when recovering from the exception.

Instead, handle this case slightly more gracefully.

The easiest way to hit this is to schedule a task for a worker that doesn't exist (or remove an existing worker, which is what I did to hit it).

Test Plan: Saw a more graceful error recovery; ran some normal successful tasks out of the queue.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11413