How do you write an error message when an Ajax call is not successful in JavaScript?

I would like to display an error message if my Ajax call is unsuccessful. Simple enough but I cannot modify the Ajax function. I created an onClick listener that does a few things when my AJAX call is successful. I would just like to do something when the call is unsuccessful.

So If I decide to add:

 if (xhr.status !== 200) {
   //Error Message here
   }

Where would it be placed?

JSFiddle Example Here

My html is:

<div id="test-form">
      <h2>Add a User:</h2>
      <label for="Username:">
       <input id="username" type="text" name="username" placeholder="Type username here">
     </label>

      <label for="Email:">
      <span id='result'></span>
      <input id="email" type="email" name="email" placeholder="email">
      </label>
      <button id="myButton">add user</button>
      <h2 class="clear">Users:</h2>
      <ul id="users"></ul>
    </div>

My code is:

 var el = document.getElementById("myButton");
    el.onclick = function() {
      addUser(username, email, onSuccess);
    }

    function onSuccess(result){
      var filter = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;

      var username = document.getElementById("username").value;
      var email = document.getElementById("email");
        var message = document.getElementById("result").classList;

      document.getElementById("users").innerHTML+= '<li>' + username +'</li>';

    if (!filter.test(email.value)) {
        document.getElementById('result').innerHTML+='Not a valid email';
        email.focus;
         return false;
     } else {


        message.add("hide");
      }

    }



    // Do not modify this function. Add user service wrapper.
    function addUser(username, email, callback) {
        var xhr = new XMLHttpRequest();
        var response;
        var success = (!!Math.round(Math.random()));

        if (!success){
            response = JSON.stringify({
                success: success,
                error: "Oups, something went wrong!"
            });
        } else {
            response = JSON.stringify({
                success: success,
                user: {
                    username: username,
                    email: email
                }
            });   
        }

        xhr.open("POST", "/echo/json/");
        xhr.onload = function () {
                if (xhr.status === 200) {
                    callback(JSON.parse(xhr.responseText));
            }
        }
        xhr.send("json=" + response);
    };