Fix bad return of ServiceUnavailable exception
Fix bad return of ServiceUnavailable exception

The ServiceUnavailable exception in StorageHealthCheck was not raised correctly which resulted in `KeyError: ServiceUnavailable('Unknown exception',)` exception when DefaultFileStorageHealthCheck failed.
pythonvietnam/django-health-check,KristianOellegaard/django-health-check,KristianOellegaard/django-health-check,clearclaw/django-health-check,kingctan/django-health-check,mspeedy/django-health-check,pythonvietnam/django-health-check,kingctan/django-health-check,mspeedy/django-health-check,clearclaw/django-health-check