Send an unauthenticated error to RPC callers when no/unmatched credentials are provided

[oxarbitrage]

The rpc.enable_cookie_auth config option is on by default but when a user try to make a call to the server there is no error message saying that the request is not authenticated but an empty reply.

$ curl --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "getinfo", "params": [] }' -H 'Content-type: application/json' http://127.0.0.1:8232/ | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    71    0     0  100    71      0  96336 --:--:-- --:--:-- --:--:-- 71000
curl: (52) Empty reply from server

I think we had this working before but we broke it at some point.