agent/exec/container: Tolerate errors from PluginList
agent/exec/container: Tolerate errors from PluginList

This endpoint may not always be implemented or supported.

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

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