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);
}
Add event listener to `el`, `fn()`.

@param {Element} el
@param {Function} fn
@api private