Mark new wallet addresses as belonging to account.

This adds some missing bookkeeping which broke rescans for the root
addresses of newly created wallets (not wallets read from disk) by
marking all active addresses (which would be just the root address) as
belonging to the account with a some particular name.

While here, return an error to the caller rather than just logging the
error if the newly-created wallet cannot be immediatelly written to
disk.