Improve Error Handling when MQTT connection fails

[vhi-di]

At the moment, the server returns a 500 server error when the connection fails.
This should be handled better by returning a 400 bad request with the following response object:

{"status":"CONNECTION ERROR","message": "some error that was the cause of the failure"}

This should then be shown in the error overlay.
For now, we can just return the raw error message without regard for translation or transformation.