[Authentication] Update error codes

[Stef Walter]

I'm suggesting some error code changes after my implementation
experience. Patch attached which includes:

 * Remove AlreadyExists. No longer needed.

 * Remove NotSupported in favor of the one defined in DBus
   itself: org.freedesktop.DBus.Error.NotSupported

 * There's no real NoSuchObject error code defined in DBus itself,
   so defined our own.

Cheers,

Stef
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