worker crashes on missing task_def

[sphenlee]

If a worker tries to execute a task and the task_def is not returned from the server (ie. 404 or some other error) the worker terminates. The work loop ends and is restarted, but the bad task is still in the queue and will keep causing work loops to end until the circuit breaker ends the process.

Missing task_def should cause the worker to update the task to error status and continue processing other tasks.