Finding no value via readable_identifiers should probably cause a failure

[balhoff]

If the ontology passed in doesn't have an annotation axiom that can provide a readable identifier with which to construct a label (for example), dosdp-tools can:

1. Default to IRI text (current behavior).
2. Default to IRI text and log a warning (at the very least should move to this).
3. Throw an error and abort (would help prevent bad pipelines).

@dosumis and @cmungall thoughts?

[balhoff]

Current behavior produces output like this: AnnotationAssertion(<http://purl.obolibrary.org/obo/IAO_0000115> <http://purl.obolibrary.org/obo/EO_0001009> "A treatment involving the use of http://purl.obolibrary.org/obo/CHEBI_24834"^^xsd:string)