code: func ErrorHandlerSet(xu *xgbutil.XUtil, fun xgbutil.ErrorHandlerFun) {
  	xu.ErrorHandler = fun
  }
docs: // ErrorHandlerSet sets the default error handler for errors that come
  // into the main event loop. (This may be removed in the future in favor
  // of a particular callback interface like events, but these sorts of errors
  // aren't handled often in practice, so maybe not.)
  // This is only called for errors returned from unchecked (asynchronous error
  // handling) requests.
  // The default error handler just emits them to stderr.