Used internally to decrement `backend`s current and total task counts 
        when `backend` could not be reached.

def _canceling_task(self, backend):
        """
        Used internally to decrement `backend`s current and total task counts 
        when `backend` could not be reached.
        """
        with self.backend_mutex:
            self.backends[backend] -= 1
            self.task_counter[backend] -= 1