ajax redirect to exception/error pages

Hi all,

When I use ajax and it returns an error, This is working great and as expected, but how do i let CodeIgniter deal with the returned error?
I looked in the manual and googled, but not sure what it is I am looking for!

error: function(xhr, status, error) {
    if(xhr.status == 400) {
        //redirect to 400 page!
    }

How Do i pass this on so CodeIgniter shows its exception page(in dev)? (or any custom page in prod 400,500 etc)