Explicit error message when setting active a Viewport that is already active.

Co-authored-by: Hugo Locurcio <[email protected]>
(cherry picked from commit e3be0913d73bdc1ff45acef30209df519c0eff2a)

servers/visual/visual_server_viewport.cpp

@@ -411,7 +411,7 @@ void VisualServerViewport::viewport_set_active(RID p_viewport, bool p_active) {
 	ERR_FAIL_COND(!viewport);
 
 	if (p_active) {
-		ERR_FAIL_COND(active_viewports.find(viewport) != -1); //already active
+		ERR_FAIL_COND_MSG(active_viewports.find(viewport) != -1, "Can't make active a Viewport that is already active.");
 		active_viewports.push_back(viewport);
 	} else {
 		active_viewports.erase(viewport);