Add Backbone.wrapError and Backbone.wrapSuccess methods

[eastridge]

Added per the discussion in: #2031

Note that I choose to add the methods directly on the Backbone object as Backbone.sync is frequently overridden.

[tgriesser]

Another option to consider, if adding additional functions on the Backbone object isn't desired:

...    
// Make the request, allowing the user to override any Ajax options.
var xhr = options.xhr = Backbone.ajax(_.extend(params, options), success, error);
...

// Set the default implementation of `Backbone.ajax` to proxy through to `$`.
Backbone.ajax = function(params, success, error) {
  return Backbone.$.ajax.call(Backbone.$, params);
};

so that someone could still access the original success & error functions by wrapping/replacing Backbone.ajax

[eastridge]

I'm not keen on that particular solution since it would require detecting which position the arguments are in (as to support jQuery.ajax(url, options)). Open to it if others are opposed to cluttering the Backbone object.

[jashkenas]

Please elaborate on the use case that we need these hooks for.

[eastridge]

@jashkenas Backwards compatibility with 0.9.2 so we can prevent an error event from being triggered in certain cases (we have a difficult to work with service layer). See:

see #2031

[braddunbar]

In what cases do you want to ignore error events?

[eastridge]

It turns out we are going to completely override sync. If this is the core team's preference for how edge cases such as ours would be handled go ahead and close out. I'd like to add though that this behavior was possible to override in 0.9.2 and we found it quite useful, so others might as well and that the removal of wrapError in 0.9.9 seems like a regression.

[braddunbar]

Backbone.wrapError was always an undocumented api, but I agree that if it was widely used we should provide a replacement. Would you mind explaining the cases in which you were ignoring error events?