You can add error-page elements to your project-specific web.xml to tell Tomcat which page to display in case an error occurs. Note that these error pages will be displayed outside of any framework context, eg the statefulness of Spring Web Flow will not be available in an error page.
Example:
<web-app ...>
<error-page>
<error-code>404</error-code>
<location>/jsp/error_404.jsp</location>
</error-page>
<error-page>
<exception-type>java.lang.Exception</exception-type>
<location>/jsp/error.jsp</location>
</error-page>
</web-app>