Health check failures do not clean up properly

[markcoatsworth]

Describe the bug
When a model job fails a health check, it moves immediately into a FailedState and hence the gateway never sends the request to shut the job down. We should send the shutdown request first. Celery logs as follows:

[2023-05-01 20:59:58,426: WARNING/ForkPoolWorker-3] The model is healthy
[2023-05-03 18:42:23,284: WARNING/ForkPoolWorker-3] Model health verification error:: HTTPConnectionPool(host='172.17.8.109', port=43537): Max retries exceeded with url: /health (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7fb5c7984b80>: Failed to establish a new connection: [Errno 111] Connection refused'))
[2023-05-03 18:42:23,285: WARNING/ForkPoolWorker-3] [2023-05-03 18:42:23,285] ERROR in models: Health check for active model OPT-6.7B failed
[2023-05-03 18:42:23,285: ERROR/ForkPoolWorker-3] Health check for active model OPT-6.7B failed
[2023-05-03 18:42:23,429: ERROR/ForkPoolWorker-3] Task tasks.verify_model_instance_health[94ca39cf-8249-46de-ae48-605aae58ec48] raised unexpected: InvalidStateError("Invalid operation for model instance state: <class 'models.FailedState'>")
Traceback (most recent call last):
File "/usr/local/lib/python3.8/site-packages/celery/app/trace.py", line 451, in trace_task
R = retval = fun(*args, **kwargs)
File "/app/gateway_service.py", line 55, in call
return self.run(*args, **kwargs)
File "/app/tasks.py", line 11, in verify_model_instance_health
model_instance.shutdown()
File "/app/models.py", line 302, in shutdown
self._state.shutdown()
File "/app/models.py", line 200, in shutdown
raise InvalidStateError(self)
errors.InvalidStateError: Invalid operation for model instance state: <class 'models.FailedState'>