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

Co-authored-by: Hugo Locurcio <[email protected]>

servers/rendering/renderer_viewport.cpp

@@ -694,7 +694,7 @@ void RendererViewport::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.");
 		viewport->occlusion_buffer_dirty = true;
 		active_viewports.push_back(viewport);
 	} else {