windowManager: Return failure to start X11 services

windowManager: Return failure to start X11 services

If for some reason the xserver failed to start, mutter/gnome-shell
should not attempt to connect to the X11 display otherwise it will
lock up in XCB.

An indication of such a premature failure of the xserver is when the X11
services fail to start.

Return the status so that the caller can cancel the connection in time
and avoid the lockup of mutter/gnome-shell in case of failure.

This, however, makes the X11 services a critical component to start
Xwayland, meaning that a failure to start those services for any other
reason than the xserver failing to start would still prevent Xwayland
and therefore X11 clients to run in Wayland. This is however a lesser
issue than mutter/gnome-shell locking up.

This basically reverts commit a96753f0 - "windowManager: X11 can work
without gsd-xsettings".

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1837>