Show error messages

[mdoering]

When a bad request is made and a 400 error returned there is no way to understand why the request was wrong.
Status 400 is probably also not sth users do understand.

For example here:
https://www.checklistbank.org/catalogue/3/sector?limit=100&mode=merge&nested=true&offset=0

The API returns a 400 response together with this message:

"message": "A nested sector search requires either an ID or NAME parameter"

The message normally exists for all errors, not just bad requests.