| DoIfErr(Action) | 
            Runs action() only if IsErr; and returns itself back.
            This is the counterpart of the Do(Action) method.
             C#  | 
| DoIfErr(ActionString) | 
            Runs action(error-message) only if IsErr; and returns itself back.
            This is the counterpart of the Do(Action) method.
             C#  |