'Raise an error if the item is NOT owned by user, otherwise return the item.
:raises exceptions.ItemAccessibilityException:'

def error_unless_owner(self, item, user, **kwargs):

    if self.is_owner(item, user, **kwargs):
        return item
    raise exceptions.ItemOwnershipException(('%s    is    not    owned    by    user' % self.model_class.__name__))
