如何获取要在HTML上显示的断言错误消息?

I've been trying to learn how to use expect.js assertion plugin for my project. My problem is i want to get the error message which only displayed on console section of my browser's inspect element to be alerted into my HTML page when i click my button. Here's what i've been doing so far

<body>
    <script>mocha.setup('bdd')</script>
    <div id="mocha"></div>
    <ol>
        <li>tes</li>
        <li>tes</li>
        <li>tes</li>
        <li>tes</li>
        <label></label>
        <div class="post">

        </div>
        <button id="klik">Klik</button>
    </ol>

    <script type="text/javascript">
    var salah="";
    describe("Index", function () {
        it("should have div",function(){
            $expect('input').to.exist(function (willThrow) {
            // Some cleanup code.
                return 'Please add a input to the page.';
            });
        });
        it("should have img",function()
        {
            $expect('img').to.exist('Please add a img to the page!');
        });
        it("should have li 4",function()
        {
        $expect('ol > li').to.have.items(4);
        });
    });
    mocha.run();

      var fn = function(){
            var missingPorHMsg = "Your div should surround the h2 and two p's";
      $expect('div.post').to.exist('Make sure your new div has a class name of "post"!')
               .and.to.have.children('h2', missingPorHMsg)
               .and.to.have.children('p', missingPorHMsg)
                            .that.has.elements(4, missingPorHMsg).end()
               .and.to.have.css( 'border'
                               , '1px dashed #ccc'
                               , "Make sure you give your post div a border!");
    };
    $('#klik').click(function(e){
        alert(fn());
    });
    </script>
</body>

When i click the button the error will show on console when i check inspect element from my browser. What I'm trying to do is get the error message and alert the message when i click the button.