Add a cookbook version uniqueness error message

:fork_and_knife: 

This adds an entry to the English locale and also adds a spec to make sure this code path is tested. Closes #549.
I think it would be more informative to the user to say _which_ version was not unique. 

```diff
- version_not_unique: "The cookbook's version number must be unique."
+version_not_unique: "%{name} (%{version}) already exists! A cookbook's version number must be unique!"
```