Dispatcher is the main point of control which dispatch request execution to given actions. It is used by filters / servlet / listener / portlet to set up and run Struts environment.
When exception occurs (misconfiguration or some internal error), Dispatcher will pass handling to DispatcherErrorHandler which by default uses HttpServletResponse#sendError to report error back to client. It will be 404 or 500 by default (depends on exception type).
If you need a different type of handling you can implement your own DispatcherErrorHandler and register it with Dispatcher using extension point struts.dispatcher.errorHandler, see Plugins for how to register your own implementation of internal mechanism.