This shouldn't happen frequently, but is just a sign that the source is
being replaced by something else. Just keep the warning for other possible
error situations.
Also, plug the potential GError leak.
Closes: https://gitlab.gnome.org/GNOME/mutter/issues/598
This code takes care of both setting up X11 selection sources whenever
X11 clients claim selection ownership, and claiming selection ownership
on a mutter X11 window whenever other selection sources claim ownership.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/320