Right now if Xwayland crashes, we crash, too.

On some level that makes sense, since we're supposed to control the
lifecycle of Xwayland, and by it crashing we've lost that control.

But practically speaking, the knock-on crash adds noise to the logs,
bug trackers, and retrace servers that only makes debugging harder.
And the crash isn't something mutter can "fix", since it's
ultimately from a bug in Xwayland anyway.

This commit makes mutter exit instead of crash if Xwayland goes away
unexpectedly.