Add Lua method to check whether a scenegraphnode is present in the scene

Make use of the function to guard mark_interesting_nodes against a nonexisting scenegraph node

scripts/scene_helper.lua

@@ -1,5 +1,7 @@
 mark_interesting_nodes = function(nodes)
     for _, n in pairs(nodes) do
-        openspace.addTag(n, "GUI.Interesting")
+        if openspace.hasSceneGraphNode(n) then
+            openspace.addTag(n, "GUI.Interesting")
+        end
     end
 end