code: 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;
      }
docs: Checks the default XML schema validation rules.<p>
  
  These rules should only be tested if this is not a test for warnings.<p>
  
  @param cms the current users OpenCms context
  @param value the value to validate
  @param valueStr the string value of the given value
  @param errorHandler the error handler to use in case errors or warnings are detected
  @param isWarning if true, this validation should be stored as a warning, otherwise as an error
  
  @return the updated error handler