docs: protected CmsXmlContentErrorHandler validateValue(
          CmsObject cms,
          I_CmsXmlContentValue value,
          String valueStr,
          CmsXmlContentErrorHandler errorHandler,
          boolean isWarning) {
  
          if (isWarning) {
              // default schema validation only applies to errors
              return errorHandler;
          }
  
          String message = null;
          if (value instanceof I_CmsXmlValidateWithMessage) {
              CmsMessageContainer messageContainer = ((I_CmsXmlValidateWithMessage)value).validateWithMessage(valueStr);
              if (null != messageContainer) {
                  message = messageContainer.key(OpenCms.getWorkplaceManager().getWorkplaceLocale(cms));
              }
          } else {
              if (!value.validateValue(valueStr)) {
                  // value is not valid, add an error to the handler
                  message = getValidationMessage(cms, value, value.getTypeName(), valueStr, true, false);
              }
          }
          if (null != message) {
              errorHandler.addError(value, message);
          }
  
          return errorHandler;
      }