Ensure concept-map worker has DBpedia Spotlight dependency fulfilled

[ralf-berger]

Observed behaviour

coursemapper-kg/concept-map depends on an external instance of DBpedia Spotlight.

If the coursemapper-kg/concept-map service is started without a DBpedia Spotlight endpoint configured, it will silently fall back to a hardcoded URL used during development. During runtime it will then continuously try to query the non-existent dependency instead of surfacing the issue.

Desired behaviour

• On startup, the coursemapper-kg/concept-map service must check if DBPEDIA_SPOTLIGHT_URL is set and terminate otherwise.
• On startup, coursemapper-kg/concept-map should validate that DBPEDIA_SPOTLIGHT_URL points to a valid DBpedia Spotlight endpoint.
• During runtime, coursemapper-kg/concept-map must terminate if the DBpedia Spotlight dependency becomes unavailable.