Unlock IRIs when an error occurs while parsing.

This is supposed to fix #982.

As for now, it's not a complete solution. The only change is: Whenever an Exception is raised during parsing, all the locked (by IRI) ontologies that are part of the parsing process are unlocked again (in the mutex) and the Exception is re-raised.

I tested this on develop.ontohub.org and the AlreadyProcessingError seems to occur less frequently *and* the user can simply retry the failed ontology which resulted in a `done` ontology every time I tried.

Since the error still occurs sometimes, there is still another problem in the concurrency handling. I'm open to suggestions.