Improve error callback for Backbone 1.0.0

This is a bit of a tricky one and I would like to hear your thoughts on this. As I was developing my app I noticed that the `error` callback arguments don't contain the `xhr` object but rather the model twice.

It seems that Backbone only expects one argument from the error callback of `$.ajax` as can be seen here: https://github.com/documentcloud/backbone/blob/1.0.0/backbone.js#L1565

To support this argument you can see that the paginator also only accepts one argument being passed to `error`: https://github.com/addyosmani/backbone.paginator/blob/master/lib/backbone.paginator.js#L851

What do you think?