Make pod fail if not connecting to RabbitMQ

As a support engineer, when deploying using kubernetes, I would like blueapi to fall over quickly if it is configured to use a message bus but is unable to connect, so that kubernetes eventual consistency algorithms can get both services into a working state as quickly as possible.

See #459 for detailed discussion.

## Acceptance Criteria
- `blueapi serve` raises an error and closes if configured to use a message bus (via `stomp` configuration) but unable to connect
- It continues to work if _not_ configured to use a message bus