another newbie question

Question

i have set up different folders which can only be accessed by specified roles like admin/customer/employee.

now whenever a user click on a button that redirects to a page where he doesnt have an access to, an error page comes up.

 now how do i fix that to redirect to my own error page?

in short heres what i want do to

 if access fails {redirect to my custom page instead of the error page}.

thanks in advance.