xside: ignore failed XUnmapWindow on non-existing window

It looks like something destroys application windows before GUI daemon
do it. Probably some X requests are reordered and XDestroyWindow got
handled before XUnmapWindow.
Anyway this is rather hard to find actual cause and since unmapped
window in most cases will be destroyed in a moment, simply ignore this
error. If this window was needed and was destroyed unintentionally, gui
daemon will notice it on the next request (this time not ignoring the
error).

Fixes QubesOS/qubes-issues#2085

gui-daemon/xside.c

@@ -217,7 +217,8 @@ int x11_error_handler(Display * dpy, XErrorEvent * ev)
 {
 	/* log the error */
 	dummy_handler(dpy, ev);
-	if (ev->request_code == X_DestroyWindow && ev->error_code == BadWindow) {
+	if ((ev->request_code == X_DestroyWindow || ev->request_code == X_UnmapWindow)
+			&& ev->error_code == BadWindow) {
 		fprintf(stderr, "  someone else already destroyed this window, ignoring\n");
 		return 0;
 	}