Show "unknown page" error

Hello,

I'm trying to achieve that when an unknown page is called, an appropiate
error message is shown. The specific case I'm looking at is when a template
is called that has no known extention (f.e. mistyping); in that case I lose
all markup (menus, headers) and get the message "*Turbine - A Servlet
Framework for building Secure Dynamic Websites." *in the header and
footer.How can I set Turbine up to keep my layout, and show some more
ellegant
message? I'm using 2.3.3

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Regards, Bo