agent/exec/container: ignore no such container errors
agent/exec/container: ignore no such container errors

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>

yongtang/swarmkit,AliyunContainerService/swarmkit,fate-grand-order/swarmkit,docker/swarmkit,yp-engineering/swarmkit,cheyang/swarmkit,AliyunContainerService/swarmkit,diogomonica/swarmkit,jlhawn/swarmkit,deljuven/swarmkit,yp-engineering/swarmkit,Noyan759/swarmkit,yongtang/swarmkit,docker/swarmkit,alyyousuf7/swarmkit,runshenzhu/swarmkit,elifa/swarmkit,aaronlehmann/swarmkit,cheyang/swarmkit,allencloud/swarmkit,sungwonh/swarmkit,dperny/swarmkit-1,cezarsa/swarmkit,aaronlehmann/swarmkit,allencloud/swarmkit,jlhawn/swarmkit,runshenzhu/swarmkit,vasil-yordanov/swarmkit,fate-grand-order/swarmkit,sungwonh/swarmkit,Noyan759/swarmkit,cezarsa/swarmkit,elifa/swarmkit,RenaudWasTaken/swarmkit,RenaudWasTaken/swarmkit,diogomonica/swarmkit,vasil-yordanov/swarmkit,dperny/swarmkit-1,alyyousuf7/swarmkit,deljuven/swarmkit