Data Provider: improve error message if ontology does not exist

Currently, if you use an ontology for dereference, for example, that is not known to the front end, you get this critical error:

`Cannot read properties of undefined (reading 'url')`

It would be better to have a clear error message (like “Cannot find ontology: sec”).

The issue does not name a file or test. Start by tracing the Data Provider path that handles ontology dereferencing and reproduce the unknown-ontology case, such as `sec`. Done means replacing the critical undefined-property error with a clear message identifying the missing ontology.

Written by the indexing model from the issue text.