gdbusconnection: Avoid tripping assertion if we fail to authenticate twice

If g_bus_get_sync() fails in authentication (because e.g. the process
uid, doesn't match the expected in EXTERNAL), a secondary call to
g_bus_get_sync() would notice we aren't initialized, and try
to initialize.

The assertion here is just wrong; we now explicitly and clearly handle
both cases where we already have an error, or we already succeeded.

https://bugzilla.gnome.org/show_bug.cgi?id=635694