Allow additional information in error responses

[fscheel]

Currently, errors only return an HTTP error number (like 404), but no body at all. Some services may fail on multiple levels to find data in the database (like get /hfc/{submissionId}/cohort may fail to find the submission or it may fail to find an associated cohort). There is no way to convey which part could not be resolved.

[fscheel]

With #83 I went a different way: The service will now return a 404 only, if the submission is not found. If there is the specified sub resource is not found, the service will return 200 with no body.

I'd like to use 204 (no content), but swagger only allows 1 Code of the 2XX family per response.