When a failed link goes to a property, it falls back to the default instead of showing an error

Summary: Noticed by a player in ARC. There are several places in Aiden O'Malley's character sheet that refer to M Caoilinn, but that's hard to spell. So it often came out as, eg, [[M Caolinn -> Full Name]]. That should display an error, since we're trying to resolve a Property of an Unknown Tag. But in fact, it is showing the Full Name of Aiden!

This one's a little worrying: it suggests that the semantics of Tags may be slightly off. I believe that if a Tag is unknown, trying to resolve a Property on it should be an error.