compositor-drm: Exit if we failed to get drm master

We can do anything without drm master.  Also log an error in case we
fail to drop it, which will be trouble for whoever we're switching to.

src/compositor-drm.c

@@ -795,7 +795,10 @@ vt_func(struct weston_compositor *compositor, int event)
 	switch (event) {
 	case TTY_ENTER_VT:
 		compositor->focus = 1;
-		drmSetMaster(ec->drm.fd);
+		if (drmSetMaster(ec->drm.fd)) {
+			fprintf(stderr, "failed to set master: %m\n");
+			wl_display_terminate(compositor->wl_display);
+		}
 		compositor->state = ec->prev_state;
 		weston_compositor_damage_all(compositor);
 		wl_list_for_each(input, &compositor->input_device_list, link)
@@ -811,7 +814,9 @@ vt_func(struct weston_compositor *compositor, int event)
 		wl_list_for_each(output, &ec->base.output_list, link)
 			drm_output_set_cursor(output, NULL);
 
-		drmDropMaster(ec->drm.fd);
+		if (drmDropMaster(ec->drm.fd) < 0)
+			fprintf(stderr, "failed to drop master: %m\n");
+
 		break;
 	};
 }