The sub-window shows different information depending on the running mode of the application: * if the application is running in debug mode, it shows the stack trace of the exception. * if the application is running in production mode, it shows a code to report that can be found in log files. The sub-window (ErrorWindow) can be invoke through an error handler (WindowErrorHandler) or just by itself calling the "open" method. It is possible to customize the message shown. This add-on is a server-side-only component, so there's no need to recompile the widgetset.