Minor fixes and guard clauses

TODO: Bootstrap ms-* and ps-* can't account for all possible levels of indentation/nesting...

js/scene.js

@@ -93,10 +93,10 @@ function loadNodes(nodes) {
             AppState.clipping.boundingSphere = node.getBound();
         }
 
-        if (!AppState.clipping.boundingSphere) {
-            console.error("There is no computed bounding sphere, clipping will fail. Missing main node?");
-        }
-
         AppState.nodes.push({id: n.label, active: true});
     });
+    
+    if (!AppState.clipping.boundingSphere) {
+        console.error("No bounding sphere computed, clipping will fail. Ensure one node has 'isMain: true'.");
+    }
 }