Return http 500 if agent uses invalid API key and is not unenrolled

[juliaElastic]

In telemetry we are seeing a lot of occurrences of 400/401 errors which are caused by unenrolled agents making requests with invalid API keys.
In order to filter out this noise, we want to have a different error code (500) for those cases where the agent is not unenrolled but still using an invalid API key.

Example:

apikey auth response bp2r2HoBy7L5RrZVC-mf: [401 Unauthorized] {"error":{"root_cause":[{"type":"security_exception","reason":"unable to authenticate with provided credentials and anonymous access is not allowed for this request","additional_unsuccessful_credentials":"API key: unable to find apikey with id bp2r2HoBy7L5RrZVC-mf","header":{"WWW-Authenticate":["Basic realm=\"security\" charset=\"UTF-8\"","Bearer realm=\"security\"","ApiKey"]}}],"type":"security_exception","reason":"unable to authenticate with provided credentials and anonymous access is not allowed for this request","additional_unsuccessful_credentials":"API key: unable to find apikey with id bp2r2HoBy7L5RrZVC-mf","header":{"WWW-Authenticate":["Basic realm=\"security\" charset=\"UTF-8\"","Bearer realm=\"security\"","ApiKey"]}},"status":401}

[juliaElastic]

Duplicate of #2861