pubsub.vala

class ErrorHandler {
    public signal void publish (Error err);
    private static ErrorHandler _default;
    public static ErrorHandler get_default () {
      if (_default == null) {
              _default = new ErrorHandler ();
      }
      return _default;
    }
}

// from the main window

ErrorHandler.get_default ().publish.connect ((err) => {
   // handle error
});

// from the GtkBox

ErrorHandler.get_default ().publish (new IOError.FAILED (""));