optimisation: stop sending gossip on dead connections
optimisation: stop sending gossip on dead connections

An error in SendProtocolMsg indicates that the connection is
permanently broken. So there is no point continuing to attempt to send
gossip on it.

The connection will eventually be removed from LocalPeer, and any
associated GossipSenders will be gc'ed, but that doesn't happen
straight away, so this change is about not doing unnecessary work in
the meantime.

tomwilkie/weave,dpw/weave,weaveworks/weave,n054/weave,n054/weave,tomwilkie/weave,rade/weave,dpw/weave,tomwilkie/weave,weaveworks/weave,rade/weave,dpw/weave,weaveworks/weave,rade/weave,dpw/weave,tomwilkie/weave,n054/weave,paulbellamy/weave,paulbellamy/weave,n054/weave,weaveworks/weave,rade/weave,dpw/weave,paulbellamy/weave,rade/weave,paulbellamy/weave,n054/weave,tomwilkie/weave