client/finality-grandpa/communication: Error on gossip engine finished

Have `<NetworkBridge as Future>::poll` return `Poll::Ready(Err)` instead
of `Poll::Ready(Ok)` to be consistent with the handling of the neighbor
packet worker stream and the gossip validator report stream. Both `Err`
as well as `Ok` shut down the `NetworkBridge` as well as the
`VoterWorker`.