installation: fixtures exception on missing models

Functionality from https://github.com/inveniosoftware/invenio/pull/1974 which is useful even after resigning from the core functionality of the mentioned PR.

Includes:
- PEP257 improvements
- checking if every fixture to be loaded has a corresponding model
What exactly do you mean? I tried it on `Collection` model and that's the result in python console:

```
from invenio.modules.search.models import Collection
>>> Collection.__subclasses__()
[]
>>> from invenio.modules.search.fixtures import CollectionData
>>> CollectionData.__subclasses__()
[]
>>>
```