Related to https://github.com/apache/airflow/issues/9655

It would be nice to add a message when you get this error with some info, like 'This probably is because a lack of RAM' or something like that. 

I have found the code where the -9 is assigned but have no idea how to add a logging message. 

        self.process = None

        if self._rc is None:
            # Something else reaped it before we had a chance, so let's just "guess" at an error code.
            self._rc = -9
tests/task/task_runner/test_standard_task_runner.py
"""
        Test that ensures that clearing in the UI SIGTERMS
        the task
        """
        path = "/tmp/airflow_on_kill"
        try:
            os.unlink(path)
        except OSError:
            pass
        dagbag = models.DagBag(
            dag_folder=TEST_DAG_FOLDER,
            include_examples=False,
        )
        dag = dagbag.dags.get('test_on_kill')
        task = dag.get_task('task1')
        session = settings.Session()
        dag.clear()
        dag.create_dagrun(
            run_id="test",
            state=State.RUNNING,
            execution_date=DEFAULT_DATE,
            start_date=DEFAULT_DATE,
            session=session,
        )