When a client receives `NotPrimaryMaster` from a secondary master, the
situation is similar as when we receive an error: the other node tells,
don't connect with me, connect with someone else. Finally the peer
even closes the connection.

Due to this similarity in structure (& because it helps us later to
teach NEO/go to correctly handle `NotPrimaryMaster` with minimal
changes), we implement `Error` for `NotPrimaryMaster`.