distinct query without "fieldname" leads to 500

Reproduce

Make a distinct-query to docstore without "fieldname".

POST http://<domain>:<port>/v0/query/study_groups?query_type=distinct
Content-Type: application/json

{"_filter": {"study_numbers": "somestudynumber"}}

Response:

{
  "code": 500,
  "message": "Internal Server Error"
}

Docstore log:

Traceback (most recent call last):
  File "/kuha_document_store-env/lib/python3.8/site-packages/tornado/web.py", line 1713, in _execute
    result = await result
  File "/kuha_document_store-env/lib/python3.8/site-packages/kuha_document_store/handlers.py", line 287, in post
    fieldname = self._query[Query.k_fieldname]
KeyError: 'fieldname'

Expected response would be 400: Bad Request with a validation message. Something like:

{
  "code": 400,
  "message": "Query type distinct requires 'fieldname'"
}