code: public void registerTagError(AbstractPageError error, JspTag tag)
              throws JspException
      {
          assert (error != null);
  
          // add the error to the list of errors
          if (_errors == null)
              _errors = new ArrayList();
  
          _errors.add(error);
  
          IErrorReporter er = getErrorReporter(tag);
          if (er == null) {
              error.errorNo = -1;
              return;
          }
  
          // add the error to the ErrorReporter tag
          er.addError(error);
          assert (error.errorNo > 0);
      }
docs: This method will add an error to the errors begin tracked by the tag. After the first time this method
  is called, <code>hasErrors</code> will return true.
  @param error The <code>EvalErrorInfo</code> describing the error.