BaseClient.ObserveChannels: handle broken connection paths
BaseClient.ObserveChannels: handle broken connection paths

Previously, if someone passed an invalid connection path to
ObserveChannels, tp-glib wouldn't pass the call up to the application,
but nor would it return an error from the method.

This manifested itself in the logger failing to return (or indeed log
messages) when I hit
<https://bugs.freedesktop.org/show_bug.cgi?id=32184>.

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib