Refactor logging in celery router

Send logging for celery router tasks to the 'rapidsms' logger rather than the 'celery' logger, and make sure celery knows the task failed by re-raising the exception.
Revert to using get_task_logger

We should still re-raise the exception, but it turns out `get_task_logger` does return a logger for the given module (not one inside celery).
Merge pull request #421 from tobiasmcnulty/develop

Refactor logging in celery router