raft: Return a more useful error message when quorum is lost
raft: Return a more useful error message when quorum is lost

When there hasn't been a leader for an extended period of time, assume
that the swarm has lost quorum. Return a more specific error message
immediately from RPCs instead of just letting RPCs timeout waiting for a
leader to be elected.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

RenaudWasTaken/swarmkit,docker/swarmkit,dperny/swarmkit-1,sungwonh/swarmkit,jlhawn/swarmkit,AliyunContainerService/swarmkit,alyyousuf7/swarmkit,dperny/swarmkit-1,alyyousuf7/swarmkit,jlhawn/swarmkit,diogomonica/swarmkit,RenaudWasTaken/swarmkit,sungwonh/swarmkit,diogomonica/swarmkit,Noyan759/swarmkit,Noyan759/swarmkit,docker/swarmkit,AliyunContainerService/swarmkit,cheyang/swarmkit,cheyang/swarmkit