Remove JobCompletedError

If there are thousands of tasks in the queue, and the user hits CTRL-C, she is flooded by JobCompletedErrors, which are pretty annoying and essentially useless. This PR also improves the situation with the revoke command,i.e. it avoids sending useless revoke messages to tasks which have been already executed, see https://bugs.launchpad.net/oq-engine/+bug/1248412