You need to include an error page element in your web.xml that handles the exceptions you choose, which should prevent Tomcat from displaying the stack trace to your page viewers when things go bad. The error-page will navigate the user to whatever page you specifiy.
Java Servlet 2.5 Specification
Download the 2.5 Final Release and read page 76, or pdf page 70 where it talks about configuring error pages.
Then read page 142/136 where is shows you what the error-page element contains.
On page 153 is an example.
You might also want to visit this page:
JavaServer Faces 1.2 API and look at the FacesException.