A simple little script I use for performing downloads via an iframe, and having control over errors (rather than a file not found screen)

iframe_downloader.js

var downloadFile = function(href){
    if (!window.download_iframe) {
        window.download_iframe = document.createElement('iframe');
        window.download_iframe.style.display = 'none';
        window.download_iframe.onload = function () {
            var doc = (window.download_iframe.contentWindow || window.download_iframe.contentDocument);
            if (doc.document) doc = doc.document;
            var content = doc.body.innerText || doc.body.textContent;
            if (content.length > 0) {
                var msg;
                try {
                    var ex = JSON.parse(content);
                    msg = ex.Message || ex.ErrorMessage || 'An error has occurred trying to download file';
                } catch (e) {
                    msg = 'An error has occured trying to download file';
                }
                // Show error, the error message is inside variable 'msg'
            } else {
              // Show success if you want?
            }
        };
        document.body.appendChild(window.download_iframe);
    }
    window.download_iframe.src = href;
};