code: function add(el, fn){
    el.addEventListener('load', function(_, e){ fn(null, e); }, false);
    el.addEventListener('error', function(e){
      var err = new Error('script error "' + el.src + '"');
      err.event = e;
      fn(err);
    }, false);
  }
docs: Add event listener to `el`, `fn()`.
  
  @param {Element} el
  @param {Function} fn
  @api private