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
```python
from invenio.ext.sqlalchemy import db
db.Model.__subclasses__()
```