ontodoc: Expect is_instance_of property to be iterable

[CasperWA]

Trying to run ontodoc for the new domain-battery locally ends up with a bug in ontodoc.py where it is running through entities, checking instances in it and iterating over an item's is_instance_of property, expecting it to be iterable. However, in the local case, it is not, which raises a TypeError.
This can be remedied by introducing a try/except test, passing for TypeError.