You would not. The first thing. Anyone could "view source" and see the error. Error messages often contain lots of information, sometimes sensitive, sometimes just revealing too much about the script and the server. So you never want to send the details to the users.

I understand though that you'd like to be able to see them easily when it's you or your coleagues accessing the site. What I do (sorry, not using HTML::Mason) is I send the details only to people who have ... well ... something set in their browser that gets sent to the server and allows me to distinguish between the developers and admins and the ordinary users. Not 100% since I'm using plain HTTP, but IMHO good enough for me.