BibClassify: error handling and PEP8

* Adds a new exception type TaxonomyError when there is a problem
  reading the ontology passed to legacy BibClassify.

* Performs some PEP8 fixes across ontology_reader.py and engine.py.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
Reviewed-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>