daemon/stats: fix notRunningErr / notFoundErr detected as unused (false positive)

Also looks like a false positive, but given that these were basically
testing for the `errdefs.Conflict` and `errdefs.NotFound` interfaces, I
replaced these with those;

    daemon/stats/collector.go:154:6: type `notRunningErr` is unused (unused)
    type notRunningErr interface {
         ^
    daemon/stats/collector.go:159:6: type `notFoundErr` is unused (unused)
    type notFoundErr interface {
         ^

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/stats/collector.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/container"
+	"github.com/docker/docker/errdefs"
 	"github.com/docker/docker/pkg/pubsub"
 	"github.com/sirupsen/logrus"
 )
@@ -131,7 +132,7 @@ func (s *Collector) Run() {
 
 				pair.publisher.Publish(*stats)
 
-			case notRunningErr, notFoundErr:
+			case errdefs.ErrConflict, errdefs.ErrNotFound:
 				// publish empty stats containing only name and ID if not running or not found
 				pair.publisher.Publish(types.StatsJSON{
 					Name: pair.container.Name,
@@ -150,13 +151,3 @@ func (s *Collector) Run() {
 		time.Sleep(s.interval)
 	}
 }
-
-type notRunningErr interface {
-	error
-	Conflict()
-}
-
-type notFoundErr interface {
-	error
-	NotFound()
-}