agent/exec/container: filter out useless error
agent/exec/container: filter out useless error

During the normal course of operation, http requests may get canceled.
While listening the events API, this error was being reported when
decoding events. This should really just be shutdown with no log message
in this case.

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

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