::WS::Client::DoCall returns an empty error message if the user authentication is missing/wrong.

This issue is addressed in branch [client-http-error].

The http package error message is returned.
This is only available if http::status returns "error".
In this case, http::status returned "ok", but http::ncode is 401 "Not authorized".

The patch adds a generic routine to return an error message in all cases.