Return errors from failed FCM registration

[iFixRobots]

Google can return HTTP 200 with Error=PHONE_REGISTRATION_ERROR instead of a token. We currently treat that as a successful registration and return empty credentials.

Return an error when registration fails or no token is returned. Confirmed the provider-error path with a live request. Go build and vet pass.

[indent]

PR Summary

Hardens RegisterGCM in gcm.go so failed FCM registrations return an error instead of silently returning empty credentials. Previously Google could reply with HTTP 200 and Error=PHONE_REGISTRATION_ERROR (or no token), which was treated as success; callers got empty credentials and no error.

• Return an error when the register response contains an Error field.
• Return an error when the HTTP status is not 200 (else if so a provider Error yields the more specific message first).
• Return an error when the response contains no token.

Issues

No issues found.

CI Checks

All CI checks passed on 791ff10.