Be more consistent about log levels

LOG_ERROR should be used in places where though recoverable (or at least
something that can be handled safely), was unexpected, and may affect
the user/application.

LOG_WARNING should be used in places where it's not entirely unexpected,
is recoverable, and doesn't really affect the user/application.

libobs/obs-scene.c

@@ -310,7 +310,7 @@ obs_sceneitem_t obs_scene_add(obs_scene_t scene, obs_source_t source)
 		return NULL;
 
 	if (!source) {
-		blog(LOG_WARNING, "Tried to add a NULL source to a scene");
+		blog(LOG_ERROR, "Tried to add a NULL source to a scene");
 		return NULL;
 	}