code: public void addError(String field, String error) {
  		Assert.notNull(field, "field must not be null");
  		Assert.notNull(error, "field must not be null");
  
  		addErrors(field, Collections.singletonList(error));
  
  		addResultProperty(SUCCESS_PROPERTY, Boolean.FALSE);
  	}
docs: Adds one error message to a specific field. Does not overwrite already existing
  errors.
  
  @param field the name of the field
  @param error the error message