If you give the wrong password for a Jabber transport, sidecar.Register() still succeeds (before the plugin actually knows whether the registration has succeeded or failed). It should fail.
However, this isn't trivial to do, because it involves a presence exchange which isn't directly a reply - it'll need some sort of mechanism for expecting the presence exchange to happen, and timing out and failing if it doesn't happen within n seconds.
See also this comment in plugins/gateways.c:
/* Eventually, we'll return success from the D-Bus method call here. */