print error when the operating system is not supported

without the above commit, the following misleading error is shown instead:
```
Error: Cannot alias File[/var/lib/puppet/concat/] to ["/var/lib/puppet/concat"] at /etc/puppet/modules/concat/manifests/init.pp:199; resource ["File", "/var/lib/puppet/concat"] already declared at /etc/puppet/modules/concat/manifests/setup.pp:49
```