Fix display exception

Pedro, I see your point of consistency.

Still, this is one of those cases where I think the old-time openerp convention is going against a more sane Python convention. I confess, I already slipped in many other `Classes` :see_no_evil: 

Let's leave that one, and I'll drop a small mail to the maining list. (OT: this is a minor topic wrt others where conventions can be improved upon, I'm thinking of testing).

Thanks!