Unmounting a container that is already unmounted is OK

We should not be throwing errors because the operation we wanted
to perform is already done. Now, it is definitely strange that a
container is actually unmounted, but shows as mounted in the DB -
if this reoccurs in a way where we can investigate, it's worth
tearing into.

Fixes #4033

Signed-off-by: Matthew Heon <mheon@redhat.com>

libpod/container_internal.go

@@ -1368,7 +1368,7 @@ func (c *Container) cleanupStorage() error {
 		// error
 		// We still want to be able to kick the container out of the
 		// state
-		if errors.Cause(err) == storage.ErrNotAContainer || errors.Cause(err) == storage.ErrContainerUnknown {
+		if errors.Cause(err) == storage.ErrNotAContainer || errors.Cause(err) == storage.ErrContainerUnknown || errors.Cause(err) == storage.ErrLayerNotMounted {
 			logrus.Errorf("Storage for container %s has been removed", c.ID())
 		} else {
 			if cleanupErr != nil {