Log Xwayland startup failures

Though unlikely, this commit makes server initialization totally
explicit in the logs.

sway/server.c

@@ -196,6 +196,9 @@ bool server_start(struct sway_server *server) {
 		server->xwayland.wlr_xwayland =
 			wlr_xwayland_create(server->wl_display, server->compositor,
 					config->xwayland == XWAYLAND_MODE_LAZY);
+		if (!server->xwayland.wlr_xwayland) {
+			sway_log(SWAY_ERROR, "Failed to start Xwayland");
+		} else {
 			wl_signal_add(&server->xwayland.wlr_xwayland->events.new_surface,
 				&server->xwayland_surface);
 			server->xwayland_surface.notify = handle_xwayland_surface;
@@ -207,6 +210,7 @@ bool server_start(struct sway_server *server) {
 
 			/* xcursor configured by the default seat */
 		}
+	}
 #endif
 
 	sway_log(SWAY_INFO, "Starting backend on wayland display '%s'",