Changing Error Message from a parent cache??

We have a cache that points to another Internet Provider
When the other internet providers caching server gives an error (like can't
find a domain name or whatever)
the error messages comes from their caching server (and puts their caching
servers name etc on it (which we don't
really like) . How can I change the error messages that goes to our users
Via a redirector? or can't this be done?
Really I'd like to put a completely different page/error message

ie
ERROR
The requested URL could not be retrieved

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While trying to retrieve the URL: http://www.rfasdas.com/
The following error was encountered:
DNS name lookup failure
The system returned:
    DNS Domain 'www.rfasdas.com' is invalid: Host not found (authoritative).
This means that:
    The named host probably does not exist.
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Generated by mailto:squid/1.1.20@x.x.x      <- how do I change this?
Thanks
Craig Whitmore
iGRIN Internet