Make the whitehall:check_slugs rake task check both extra_organisation_slugs and whitehall_slug

- The `SlugsMissingException` expects a hash rather than an array, so we can print multiple missing slugs rather than just one, eg:

```
Slugs missing from Whitehall:
bis: ["made-up-slug", "second-made-up-slug"]
```

(Paired with @jennyd on this.)