Ignore ErrLayerUnknown error when attempting to cleanup container

Fixes: containers#11207

[NO TESTS NEEDED] Since I don't know how to get into this situation.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

libpod/runtime_cstorage.go

@@ -112,12 +112,14 @@ func (r *Runtime) removeStorageContainer(idOrName string, force bool) error {
 			return errors.Wrapf(define.ErrCtrStateInvalid, "container %q is mounted and cannot be removed without using force", idOrName)
 		}
 	} else if _, err := r.store.Unmount(ctr.ID, true); err != nil {
-		if errors.Cause(err) == storage.ErrContainerUnknown {
+		if errors.Is(err, storage.ErrContainerUnknown) {
 			// Container again gone, no error
 			logrus.Infof("Storage for container %s already removed", ctr.ID)
 			return nil
 		}
-		return errors.Wrapf(err, "error unmounting container %q", idOrName)
+		if !errors.Is(err, storage.ErrLayerUnknown) {
+			return errors.Wrapf(err, "error unmounting container %q", idOrName)
+		}
 	}
 
 	if err := r.store.DeleteContainer(ctr.ID); err != nil {