code: public void registerTagError(String message, Throwable e)
              throws JspException
      {
          ErrorHandling eh = getErrorHandling();
          eh.registerTagError(message, getTagName(), this, e);
      }
docs: This will report an error from a tag.  The error will
  contain a message.  If error reporting is turned off,
  the message will be returned and the caller should throw
  a JspException to report the error.
  @param message - the message to register with the error
  @throws javax.servlet.jsp.JspException - if in-page error reporting is turned off this method will always
  throw a JspException.