Change Error pages

Hi all.

I have a requirement to change the standard error pages of J2EE engine, i mean change the message(page) displayed when a WebDynpro application session expires for example. The page that says:

<b>500 Internal Server Error

The Web Dynpro application XXXX has expired....

</b>

and to change the message(page) displayed when some exception is not caught. The page that says:

<b>500 Internal Server Error

Failed to process request. Please contact your system administrator.

...

Error Summary

...

Root Cause

The initial exception that caused the request to fail, was:...

</b>

I wanna know if there is possible to change this pages, i mean by configuration? Some place where i can say if error is X then show the error page XX?

Regards.

Gregory.