feature (log): log auth failure - #852

We want to have a warning so that it is possible from the logs to immediately
see if there have been failed logging attempts.

This might allow in the future to set up tools like fail2ban.

Co-authored-by: Marco Seravalli <me@marcoseravalli.com>

server/ctrl/session.go

@@ -307,6 +307,7 @@ func SessionAuthMiddleware(ctx *App, res http.ResponseWriter, req *http.Request)
 	// - identity provider redirection uri. eg: oauth2, openid, ...
 	templateBind, err := plugin.Callback(formData, idpParams, res)
 	if err == ErrAuthenticationFailed {
+		Log.Warning("failed authentication - %s", err.Error())
 		http.Redirect(
 			res, req,
 			req.URL.Path+"?action=redirect",