search/batch endpoint: restore batch size of 100

Issue description

The old batch endpoint allowed a batch size of 100 query objects.

The new /search/batch endpoint only allows 50 objects. If more than 50 are specified it responds with a generic 502 Bad Gateway error:

{
  "message": "Internal server error"
}

Issue request

• Please allow a batch size of at least 100 again
• Please provide a more distinct error message with a given error type that can be handled accordingly.