Invalid webwork action produces exception in Tomcat

When navigating to an invalid webwork action URL you will be presented with an exception error report and not the friendly 404 error page.

confluence 2.5.6 produces correct 404 error pages:
http://confluence.atlassian.com/foo.html - correct 404 page
http://confluence.atlassian.com/foo.action - correct 404 page

confluence extranet_2.6_20070820:
http://extranet.atlassian.com/foo.html - picked up by 404 error page correctly (although see: CONF-9194)
http://extranet.atlassian.com/foo.action - exception error page