「iframe.load()」が失敗したときに iFrame の読み込みステータスを確実に確認する方法

<code class="javascript">function checkIframeLoaded() {
    // Get a handle to the iframe element
    var iframe = document.getElementById('i_frame');
    var iframeDoc = iframe.contentDocument || iframe.contentWindow.document;

    // Check if loading is complete
    if (iframeDoc.readyState == 'complete') {
        //iframe.contentWindow.alert("Hello");
        iframe.contentWindow.onload = function() {
            alert("I am loaded");
        };
        // The loading is complete, call the function we want executed once the iframe is loaded
        afterLoading();
        return;
    }

    // If we are here, it is not loaded. Set things up so we check the status again in 100 milliseconds
    window.setTimeout(checkIframeLoaded, 100);
}

function afterLoading() {
    alert("I am here");
}

window.onload = checkIframeLoaded();</code>