gdbusconnection: Don't g_printerr() when exiting

exit-on-close for a DBus connection is a completely normal thing.  On
a regular GNOME login, gdm retains the X server, but terminates the
session login bus and associated helpers like gnome-settings-dameon,
the a11y tools, etc.

I've seen several downstream reports of confusion as to what these
apparent error messages mean in the system log.  It doesn't help
that they're so obtuse.

We're also printing them to stderr, when this is not an error.

The reason this was introduced is presumably some people were confused
as to why their process exited when the system bus did.  But the
solution for that I believe is documentation, not printing stuff to
everyone's system log in normal operation.

https://bugzilla.gnome.org/show_bug.cgi?id=742386