make ClassNotFoundException a subclass of Error

This way Classes.prototype.loadClassFile will throw something like "Uncaught ClassNotFoundException: com/nokia/mid/ui/frameanimator/FrameAnimator.class" instead of the vague "Uncaught [object Object]".

It could be. The two are equivalent, and I was considering using the Object.create variant, but I didn't see any other occurrences of Object.create in the file, whereas the `new` operator is used regularly, so I used `new` for consistency.