yabe: add some error examples (403 and 404)

http://groups.google.com/group/play-framework/browse_thread/thread/296e1de9e07bdfbf?hl=en

One thing I've found missing in the Yabe tutorial is examples of handling errors, like for instance in case of a bad post id (currently it gives a NPE). It took me some time before I've found out the notFound() method and such. Sure it was right under my nose, but for some reason I was not searching there, and thus my first solution for implementing a 404 response has been to explicitly throw a NotFound exception, because that's what I had found first in the API doc.

The attached patch is a small addition to the Yabe example:
 - add some notFoundIfNull() calls for handling bad post id
 - add some forbidden() calls for user X editing posts of user Y

I think seeing those would have helped me understand sooner that render() is not the only available result method.