windowManager: Ignore error without systemd

windowManager: Ignore error without systemd

On systems/setups not using systemd, a failure to start gsd-xsettings
with NOT_SUPPORTED does not denote a failure to start Xwayland, just
that we're not using systemd.

In that case, we should just ignore the error, otherwise it will prevent
Xwayland to start on such systems/setups without systemd.

Thanks to Rose Kunkel (@rosekunkel) for spotting the issue/suggesting
the fix.

Fixes: 019229c40e - windowManager: Return failure to start X11 services
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4284
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1850>