Show stacktrace on rejected promise.

[Carreau]

Not just error.message but error.stack in an unfoldable code tag with white-space:pre.

Or at least console-warn it to help debug.

[Carreau]

Notebook.prototype.load_notebook_error = function (error) {
        this.events.trigger('notebook_load_failed.Notebook', error);
        var msg;
        if (error.name === utils.XHR_ERROR && error.xhr.status === 500) {
            utils.log_ajax_error(error.xhr, error.xhr_status, error.xhr_error);
            msg = "An unknown error occurred while loading this notebook. " +
            "This version can load notebook formats " +
            "v" + this.nbformat + " or earlier. See the server log for details.";
        } else {

            msg = $('<div/>').append($('<p/>').text(error.message));
            msg.append($('<pre/>').text(error.stack));
        }
        dialog.modal({
            notebook: this,
            keyboard_manager: this.keyboard_manager,
            title: "Error loading notebook",
            body : msg,
            buttons : {
                "OK": {}
            }
        });

In notebook.js seem to be a start.