display-settings: Inform the user in the event of configuration application failure and update the UI accordingly
In principle, this is independent of the windowing system, but the problem doesn't seem to arise on X11. On Wayland, since a certain wlroots commit (probably this one), configuration application failure has become relatively likely, enough to justify adding a dialog (see also https://github.com/labwc/labwc/issues/1525).
That said, as wlroots is rather unstable, this may change again in the future, so I'd rather open an issue than jump into doing anything about it at the moment.
If this were to be implemented, it might be an opportunity to also make the change I'm commenting on in 72b314ac, which is related. (done)