**Apache Airflow version**: 2.0.0a2

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): v1.15.12

**Environment**:
- **OS** (e.g. from /etc/os-release): CentOS 7


**What happened**:

When using KubernetesExecutor, a single SIGTERM doesn't result in the scheduler actually exiting cleanly. However, it will exit after receiving a second SIGTERM.

The same behavior happens if you have an exception in the scheduler. An easy way to replicate it is to raise in `pod_mutation_hook`.

In practice, this means the scheduler doesn't get restarted by K8s after encountering a exception. Or, when terminating the scheduler pod, it sticks around until k8s ultimately sends it a SIGKILL after its grace period is up.

**What you expected to happen**:

The scheduler to exit after a single SIGTERM.

**How to reproduce it**:
The easiest way to reproduce it is to simply send a SIGKILL to your KubernetesExecutor scheduler pod:

```
$ kubectl exec {scheduler_pod_name} kill 1
```

Note, the pod stays running, while the logs report:

```
[2020-10-30 17:34:09,581] {scheduler_job.py:794} INFO - Exiting gracefully upon receiving signal 15
[2020-10-30 17:34:10,586] {process_utils.py:97} INFO - Sending Signals.SIGTERM to GPID 59
[2020-10-30 17:34:10,879] {process_utils.py:63} INFO - Process psutil.Process(pid=59, status='terminated', exitcode=0, started=
'17:33:23') (59) terminated with exit code 0
[2020-10-30 17:34:10,882] {process_utils.py:97} INFO - Sending Signals.SIGTERM to GPID 59
[2020-10-30 17:34:10,882] {scheduler_job.py:1329} INFO - Exited execute loop
```

Issue another `kill`:
```
$ kubectl exec {scheduler_pod_name} kill 1
```

And the pod will be restarted by k8s.

If you can't reproduce it, let me know and I'll try and get something running on kind.

**Anything else we need to know**:

The same behavior happens if you have an exception in the scheduler. An easy way to replicate it is to raise in a `pod_mutation_hook`.


tests/jobs/test_scheduler_job.py
try:
            dags_folder = mkdtemp()
            unparseable_filename = os.path.join(dags_folder, TEMP_DAG_FILENAME)
            
            with open(unparseable_filename, 'w') as unparseable_file:
                unparseable_file.writelines(UNPARSEABLE_DAG_FILE_CONTENTS)
            self.run_single_scheduler_loop_with_no_dags(dags_folder)
            
            with open(unparseable_filename, 'w') as unparseable_file:
                unparseable_file.writelines(
                    PARSEABLE_DAG_FILE_CONTENTS + os.linesep + UNPARSEABLE_DAG_FILE_CONTENTS
                )
            self.run_single_scheduler_loop_with_no_dags(dags_folder)
        finally:
            shutil.rmtree(dags_folder)
        session = settings.Session()
        import_errors = session.query(errors.ImportError).all()
        assert len(import_errors) == 1
        import_error = import_errors[0]
        assert import_error.filename == unparseable_filename
        assert import_error.stacktrace == f"invalid syntax ({TEMP_DAG_FILENAME}, line 2)"
    def test_remove_error_clears_import_error(self):