userExists and groupExists discriminates between shown objects and  obje...

...cts hid by SID

A very frequent error in the trac is that the user try to add an 'Administrator' user name or a 'Guests' group or whatever and she gets a message which sasys the user already exists but it is not shown in the interface.

Having a different message  in this cases make things clearer to people which does not know the hidden SIDs feature