Added error references to models/collections as shown in the docs.

In the docs, there is an example, which was not working because the error constructors are not attached to the models.

```
Customer.login(email, password)
  .then(function(customer) {
    res.json(customer.omit('password'));
  }).catch(Customer.NotFoundError, function() {
    res.json(400, {error: email + ' not found'});
  }).catch(function(err) {
    console.error(err);
  });
```