print the content $res->content on failure also

if ($res->is_success) {
  print $res->content;
} else {
  print $res->status_line;
  print $res->content;
}
[download]

You should get an error code like this

{"error": {"message": "HMAC authentication key and signature was given
+, but they are invalid.", "error_code": 41}}
[download]