def stop_heartbeat(self):
        if self._heartbeat_timer is not None:
            self._heartbeat_timer.stop()
            self._heartbeat_timer = None
ensure KeyError on resubmit of nonexistant task
dataset/ETHPy150Open mrjoes/tornadio2/tornadio2/session.py/Session.stop_heartbeat--crossover with--dataset/ETHPy150Open ipython/ipython-py3k/IPython/parallel/tests/test_client.py/TestClient.test_resubmit_badkey