There is a good tutorial at:
http://javascriptkit.com/howto/htaccess2.shtml
You would create something like:
ErrorDocument 400 /errors/badrequest.html
ErrorDocument 401 /errors/authreqd.html
ErrorDocument 403 /errors/forbid.html
ErrorDocument 404 /errors/notfound.html
ErrorDocument 500 /errors/serverr.html
as your htaccess file and when you get the specific error it sends you to the link e.g.
a 400 error would send you to the page /errors/badrequest.html