More gracefully handle unexpected storage deletion

We have other tools using containers/storage. They can delete our
containers in c/storage without us knowing. Try and handle this
better by warning instead of erroring when delete our storage and
it is already gone.

This does not handle cases where libpod thinks the container is
mounted, but it is not. This is harder to check for, because
c/storage Mount() and Unmount() take a layer, image, or container
and that complicates our "container no longer exists" question.
Further work is needed here.

Signed-off-by: Matthew Heon <matthew.heon@gmail.com>

Closes: #571
Approved by: rhatdan

libpod/container_internal.go

@@ -223,6 +223,15 @@ func (c *Container) teardownStorage() error {
 	}
 
 	if err := c.runtime.storageService.DeleteContainer(c.ID()); err != nil {
+		// If the container has already been removed, warn but do not
+		// error - we wanted it gone, it is already gone.
+		// Potentially another tool using containers/storage already
+		// removed it?
+		if err == storage.ErrNotAContainer {
+			logrus.Errorf("Storage for container %s already removed", c.ID())
+			return nil
+		}
+
 		return errors.Wrapf(err, "error removing container %s root filesystem", c.ID())
 	}