Modifying the "You are not logged in" message

[MadMorbius]

I'd like to modify the message that's returned to a user when they attempt to access a resource to which they have no access or are not logged in. The current message is:

%user%, you do not have permission to access this page. This could be due to one of several reasons:

(lists reasons)

I'd like to add to the reasons that are displayed. Can someone tell me where that's done?

Thanks