Changing an Error Message

Hello Folks,

I have what seems like a question that should have a very simple  
answer but unfortunately I can't find or figure out what the answer  
should be.

I was wondering how to go about changing the error message a person  
receives whenever an application/instance they are seeing is not  
available.

For example I have an application that can be accessed at:

http://my-domain.com/cgi-bin/WebObjects/MyApp.woa/1

If for instance the application went down.  Is there any way to  
change the error message so that they see "We sorry but the  
application is not available at the moment.  Please come back later"  
rather then "The requested application was not found on this server."

Thanks for your help.

Regards,
Jeff