This method has to be called *once, and only once* for each container registered in `register_container`.
        :param container_id: the container id to check
        :return: a string containing "timeout" if the container was killed. None if it was not (std format for container errors)

async def was_killed(self, container_id):
        """
        This method has to be called *once, and only once* for each container registered in `register_container`.
        :param container_id: the container id to check
        :return: a string containing "timeout" if the container was killed. None if it was not (std format for container errors)
        """
        if container_id in self._watching:
            self._watching.remove(container_id)
        if container_id in self._container_had_error:
            self._container_had_error.remove(container_id)
            return "timeout"
        return None