Edit in JSFiddle

javascript:(function(bm) {
    if (!window.bmlet) window.bmlet = {};
    if (!window.bmlet[bm]) {
        window.bmlet[bm] = true;
        var s = document.createElement('script');
        s.onerror = function() {
            window.bmlet[bm] = false;
        };
        s.src = bm;
        document.getElementsByTagName('head')[0].appendChild(s);
    }
}('https://raw.github.com/gist/2ef216215c516cdf4a0f/2137c66040360bb1c9259645f6865a3a18a37f20/bookmarklet.js'));