Introduce new exception class when an Object is not found

The GenericDao throws an ObjectRetrievalFailureException when the persistent instance of a class is not found. This exception can be raised in a few other cases too - e.g. when the request is for the wrong class. This suggestion is to create a new exception class, derived from ObjectRetrievalFailureException which is specific to the case when no such object exists: NoSuchObjectException or ObjectNotFoundException.