Improve error message for ontology missing `a owl:Ontology` triple for graphic generation

[kstudzin]

Currently when generating a schema graphic, if the ontology doesn't have a triple identifying a resource as an owl:Ontology, onto_tool throws a StopIteration error. This could be wrapped in an message that helps the user fix the problem more easily.

[sa-bpelakh]

I agree, it could be a lot more helpful.