Be more lenient on auplink errors.

On aufs, auplink is run before the Unmount. Irrespective of the
result, we proceed to issue a Unmount syscall. In which case,
demote erros on auplink to warning.

Signed-off-by: Anusha Ragunathan <anusha@docker.com>

daemon/graphdriver/aufs/mount.go

@@ -12,7 +12,7 @@ import (
 // Unmount the target specified.
 func Unmount(target string) error {
 	if err := exec.Command("auplink", target, "flush").Run(); err != nil {
-		logrus.Errorf("Couldn't run auplink before unmount %s: %s", target, err)
+		logrus.Warnf("Couldn't run auplink before unmount %s: %s", target, err)
 	}
 	if err := syscall.Unmount(target, 0); err != nil {
 		return err