[Vala] error domain constant names

["Michael 'Mickey' Lauer" <mickey vanille-media de>]

With the recent introduction of proper dbus error marshalling courtesy Didier 
Ptitjes, he and me ran into an argument about error domain constant names and 
we'd like your input.

Since the dbus error names are getting CamelCased, I think it's cleaner to use 
CamelCased names for error constants locally as well -- his position is that 
it's Vala convention to use CONSTANT_NAMED error constants.

What's your take on that?

Cheers,

Mickey & Didier.