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
If I use this method, I need to have model registry. From fixture name I can get model name, but I cannot import it because I don't know the full path. Without model registry I need to iterate over all of them to get the paths.

I rather wouldn't rewrite fixtures and models to registries by myself so I propose to add here FIXME and integrate the current solution.

The current code in this PR doesn't increase computational complexity, because there was already double 'for' loop. The difference is just additional 'if' at line 288.