Edit in JSFiddle

$.post('/echo/json/', {json:JSON.stringify({'error':true})})
    .done(function (response) {
        //Our JSON response will come back with an http status of 200,
        // so we have to handle our errors in the "done" (success)
        // callback of the promise returned by post().
        if (response.error) {
            $("#status").html("An error occurred");
        } else {
            $("#status").html("Success!");
        }
    })
    .fail(function (response) {
        //This gets executed on failing http status codes (e.g. 400)
        $("#status").html("A server error occured (failing http status code)");
    });

<div id="status"></div>