Finding no value via readable_identifiers should probably cause a failure

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?

Start by tracing how dosdp-tools handles readable_identifiers when an ontology lacks a suitable annotation axiom. Review the alternatives in the issue and determine whether the intended behavior is fallback text, a warning, or an error. Done means the chosen behavior is implemented and its effect on bad pipelines is verified.

Written by the indexing model from the issue text.