Output a useful message if `git submodule update --init --recursive` not
yet performed

```
$ ansible --version
ansible 1.8 (submodule_ansible_version 59ae596484) last updated 2014/09/28 13:20:51 (GMT +1000)
  lib/ansible/modules/core: (detached HEAD 617a52b20d) last updated 2014/09/28 13:15:25 (GMT +1000)
  lib/ansible/modules/extras:  not found - use git submodule update --init lib/ansible/modules/extras
```