Return a 500 for _ping if there is no elected primary
Return a 500 for _ping if there is no elected primary

Signed-off-by: Wayne Song <d84980645ee1831b184d4a7081acdc67737e72a7@docker.com>

jimmycmh/swarm,allencloud/swarm,fugr/swarm,allencloud/swarm,jimmycmh/swarm,fugr/swarm