Proper error message for non-existing OEO class

[l-emele]

Description of the issue

When one opens the IRI of a non-existent OEO class/individual/property, e.g. https://openenergyplatform.org/ontology/oeo/OEO_99999999/ then an almost empty page is shown:

The user does not get any feedback that she or he requested a non-existing entity.

Ideas of solution

Ontobee is a good example how to handle this: For a non-existent IRI the user gets a delievered a proper error message. For example, http://purl.obolibrary.org/obo/BFO_1234567 gets as usual[^1] redirected to https://ontobee.org/ontology/BFO?iri=http://purl.obolibrary.org/obo/BFO_1234567 but it has the following content:

[^1]: OBO IRIs always redirect to Ontobee

Workflow checklist

• [ ] I am aware of the workflow in CONTRIBUTING.md