[Exim] Custom bounce messages out of /etc/aliases

I used to be able to do this on sendmail, wondering if I can easily do
it in exim.

In the virtual user table we had something that looked like this:

usergone@???    error:nouser This person is long gone

Any ability to do that easily. I suppose I can do a router that looks
up a username in an additional alias file and substitute in the text
that it finds, but wondering if there is anyway to get local_delivery to
do it.

Cheers!

--Brett