Exposed wrapError as Backbone.wrapError

Exposing wrapError as Backbone.wrapError would allow users to define a default error handler for all ajax calls:

```javascript
    var originalWrapError = Backbone.wrapError;

    Backbone.wrapError = function(model,options){

        if(!options.error){
            options.error = function(){
                //my default error handler
            }
        }

        originalWrapError(model,options);
    };
```