[40] 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: 019229c4 - windowManager: Return failure to start X11 services Closes: #4284 (closed) Part-of: !1850 (merged)

(cherry picked from commit d9239e2c)