haproxy/examples/errorfiles
..
400.http
403.http
408.http
500.http
502.http
503.http
504.http
README

These files are default error files that can be customized
if necessary. They are complete HTTP responses, so that
everything is possible, including using redirects or setting
special headers.

They can be used with the 'errorfile' keyword like this :

    errorfile 503 /etc/haproxy/errors/503.http