improve error on invalid vars file (if its a list instead of a dict)

This branch improves the error message when a roles var file is in the wrong format (in my case, list instead of dict).

I made a mistake in my vars file and it was a list instead of a dict. This caused a somewhat cryptic error message:
```
$ cat vars/main.yml
- include: zimk_ntp.yml
$ ansible-playbook --list-tasks site.yml
[...]
AttributeError: 'list' object has no attribute 'items'
```
and it took me a bit to figure out what I did wrong. This branch should make it more obvious to users like me where to look:
```
$ ansible-playbook --list-tasks site.yml
ERROR: vars from '/home/mvo/src/ansible-config/roles/base-server/vars/main.yml' are not a dict
```

I'm happy to change the exact message if you prefer a different wording and/or provide better instructions how to reproduce. Just let me know what you need :)

Cheers,
 Michael