[xwl] Restart xwayland regardless of exit value

In the case of an assert, for example a wayland error Xwayland will exit, but with an exitStatus of "normal exit".

We also trigger a shutdown should kwin encounter an error on the X11 side, this would be a clean exit from an XwaylandLauncher point of view. It makes sense to try to restart.

Deliberate shutdown is handled by destroying the XwaylandLauncher. This means it's not an issue for final kwin teardown.