Under Jboss5-7 it was no problem to do the following in web.xml:

<error-page>
<error-code>404</error-code>
<location>/WEB-INF/errors/dynamic_error.jsp</location>
</error-page>
<error-page>
<error-code>500</error-code>
<location>/WEB-INF/errors/staticFatalError.html</location>
</error-page>

If a generic exception is now thrown from a JSF page the behavior is as follows:
8.0 : stack overflow exceptions in undertow code; a blank white page in browser
8.1 : 404 for the error template itself; white page with small 404 text in top left

If the static html and jsp are located outside the WEB-INF everything works
normally.