code: function setButter(message, error, traceback, asHtml) {
    var butter = $('#butter');
    // Prevent flicker on butter update by hiding it first.
    butter.css('display', 'none');
    if (error) {
      butter.removeClass('info').addClass('error');
    } else {
      butter.removeClass('error').addClass('info');
    }
    butter.children().remove();
    if (asHtml) {
      butter.append($('<div>').html(message));
    } else {
      butter.append($('<div>').text(message));
    }
  
    function centerButter() {
      butter.css('left', ($(window).width() - $(butter).outerWidth()) / 2);
    }
  
    if (traceback) {
      var showDetail = $('<a href="">').text('Detail');
      showDetail.click(function(event) {
        $('#butter-detail').toggle();
        centerButter();
        event.preventDefault();
      });
      var butterDetail = $('<pre id="butter-detail">').text(traceback);
      butterDetail.css('display', 'none');
  
      butter.append(showDetail);
      butter.append(butterDetail);
    }
    centerButter();
    butter.css('display', null);
  }
docs: Sets the status butter, optionally indicating if it's an error message.