Error hiding for missing credentials

[twocs]

If the credentials do not exist, MATLAB-online hides the Plotly:CredentialsNotFound error with plotlyfigConstructor:notSignedIn because of a try/catch.

https://github.com/plotly/MATLAB-Online/blob/72b22f5ce8572118e8447189f472a7ad58d78760/plotly/plotlyfig.m

            %-UserData-%
            try
                [obj.UserData.Username,...
                    obj.UserData.ApiKey,...
                    obj.UserData.PlotlyDomain] = signin;
            catch
                errkey = 'plotlyfigConstructor:notSignedIn';
                error(errkey, plotlymsg(errkey));
            end

If we are running with offline capability, it would be very exceptionally nice to not need any config/credential files at all. Encountered this when I was investigating an issue with the plotly/MATLAB-Online API ( #https://github.com/imatlab/imatlab/issues/13)