[users@httpd] how to PROMT the user who access the site that we ask for a client certificat

[Buddy wu]

 I use apache with ssl and require client cert. how to let the user know he
shouldrequest a cert? now when a person without a cert access the site, it
only appreas that "the site can't be displayed ,maybe network problem"
how to give a hint to user , he should use a cert to access this cert