Improve handling of failure to enable hotspot from NM
- Report error (and revert everything we've done so far) if NM fails to activate our hotspot connection.
- Make clients re-fetch whether hotspot is enabled if an error is reported from the service.
Could help with #95 (closed) and #96 (closed), but I'm not sure. @kethen and @RaoulKramer might be interested in testing this. Also, this complements !113 (merged).