To enable Seam's exception handling, we need to make sure we have the master servlet filter declared in web.xml:

<filter>
    <filter-name>Seam Filter</filter-name>
    <filter-class>org.jboss.seam.servlet.SeamFilter</filter-class>
</filter>

<filter-mapping>
    <filter-name>Seam Filter</filter-name>
    <url-pattern>*.seam</url-pattern>
</filter-mapping>

You may also need to disable Facelets development mode in web.xml and Seam debug mode in components.xml if you want your exception handlers to fire.