Fixtures exception on missing models

Functionality from https://github.com/inveniosoftware/invenio/pull/1974#issuecomment-49713726 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
You may be right. Actually more unreadable are 'f' and 'ff' here. 

On the other hand this is not a performance bottleneck at all, because the function is not used during normal work of the server, just to set it up and for development. May I convince you then that in such a case when speed is not important, the readability should be the priority?