Add function to get claimed nameplates

[JustusFT]

It would be nice to have a function that returns a list of claimed nameplates.

Use case:
In wormhole-william, when a user tries to receive with a code, we check if there's a claimed nameplate like so. If the nameplate is unclaimed, there's nothing to receive so the app exits with an error message Nameplate is unclaimed. We also use this error in winden to inform the user that they put in a wrong code and try again.
Currently in magic-wormhole.rs, if the user puts in a code with an unclaimed nameplate, the application hangs.