Fix errcheck: error return value of aufs.Unmount is not checked

Signed-off-by: Jan Rodák <[email protected]>

drivers/aufs/aufs.go

@@ -681,7 +681,9 @@ func (a *Driver) Cleanup() error {
 func (a *Driver) aufsMount(ro []string, rw, target string, options graphdriver.MountOpts) (err error) {
 	defer func() {
 		if err != nil {
-			Unmount(target)
+			if err1 := Unmount(target); err1 != nil {
+				logrus.Warnf("Unmount %q: %v", target, err1)
+			}
 		}
 	}()