agent/exec: use correct prefixes in errors
agent/exec: use correct prefixes in errors

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

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