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
local_task_job = mock.Mock()
        local_task_job.task_instance = mock.MagicMock()
        local_task_job.task_instance.run_as_user = getpass.getuser()
        local_task_job.task_instance.command_as_list.return_value = [
            'airflow',
            'tasks',
            'test',
            'test_on_kill',
            'task1',
            '2016-01-01',
        ]
        runner = StandardTaskRunner(local_task_job)
        runner.start()
        time.sleep(0.5)
        pgid = os.getpgid(runner.process.pid)
        assert pgid > 0
        assert pgid != os.getpgid(0), "Task should be in a different process group to us"
        processes = list(self._procs_in_pgroup(pgid))
        runner.terminate()
        for process in processes:
            assert not psutil.pid_exists(process.pid), f"{process} is still alive"
        assert runner.return_code() is not None
    def test_on_kill(self):