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
I think you can just iterate over fixtures and try to find model from `db.Model.__subclasses__()`. We have over 500 models but only ~35 fixtures.