According to
http://www.aolserver.com/docs/admin/config-reference.tcl.txt you just need to have
#
# Internal redirects
#
ns_section "ns/server/${servername}/redirects"
ns_param 404 "/notfound.html" ;# Not Found error page
ns_param 500 "/servererror.html" ;# Server Error page
in your aolserver config file. Works great for me :) As an extra, you might want to consider having the error pages send you an email or put a note into the log.