You can specify custom error messages in the HTTP server's config file
(httpd.conf)
These settings are for IHS (IBM HTTP Server) but I think the same will
work with apache servers too.
This is for error 500 (WAS is down)
If you want a simple message:
ErrorDocument 500 "The server made a boo boo."
If you want to show a custom html file:
ErrorDocument 500 /error500.html
I'm not sure that 'SRVE0255E: No virtual host/web group ...." has an
error 500 code, but you can easily find out the error code from the
HTTP server's 'access_log'
The code is in the end of every line. For example ... HTTP/1.1" 200
39 -> 200 is means transaction was successful.