Service invoked when an uncaught exception occurs. The error handler is responsible for providing a response to the
user to describe the error.
The default implementation accepts the configuration, which maps an exception class to a corresponding helper.
The helper is either an instance of ExceptionHandlerAssistant or is a Class for a page that will be redirected
to, to present the exception.
Responsible for handling the error in some way and providing some response to the client. A
default implementation may render an error response page.
The handler is also responsible for setting the response status and the X-Tapestry-ErrorMessage response header.
These are very important in Ajax requests to allow the client-side logic to detect the error and present it to