code: public function setErrorMessages($errorMessages)
      {
          if (is_array($errorMessages)) {
              $errorMessages = array_merge($this->defaultErrorMessages(), $errorMessages);
          }
          $this->_errorMessages = $errorMessages;
      }
docs: Sets the list of error messages responding to invalid filter structure, in format: `[errorKey => message]`.
  Message may contain placeholders that will be populated depending on the message context.
  For each message a `{filter}` placeholder is available referring to the label for [[filterAttributeName]] attribute.
  @param array|\Closure $errorMessages error messages in `[errorKey => message]` format, or a PHP callback returning them.