Empty error message in Model::addError().

This can be useful to make [this sample code](https://gist.github.com/resurtm/5815403) even better. If you look at the `Product::afterValidate()` method you can see `$this->addError('orders', 'Orders are invalid!');` call. With patch provided in this PR it can be rewritten as `$this->addError('orders');`, which looks much better as we don't want/need to specify error message for the `orders` relation—our purpose is to mark main model as having errors.

I and @creocoder thought about introduction of returning boolean value in the `Model::afterValidate()` method but it seems as worse approach.