code: protected String handleException (HttpServletRequest req, Logic logic, Exception error)
      {
          log.warning(logic + " failed on: " + RequestUtils.reconstructURL(req), error);
          return ExceptionMap.getMessage(error);
      }
docs: If a generic exception propagates up from {@link Logic#invoke} and is not otherwise
  converted into a friendly or redirect exception, the application will be required to provide
  a generic error message to be inserted into the context and should take this opportunity to
  log the exception.
  
  <p><em>Note:</em> the string returned by this method will be translated using the
  application's message manager before being inserted into the Velocity context.