code: @SuppressWarnings("unchecked")
  	public void addErrors(String field, List<String> errors) {
  		Assert.notNull(field, "field must not be null");
  		Assert.notNull(errors, "field must not be null");
  
  		// do not overwrite existing errors
  		Map<String, List<String>> errorMap = (Map<String, List<String>>) this.result
  				.get(ERRORS_PROPERTY);
  		if (errorMap == null) {
  			errorMap = new HashMap<>();
  			addResultProperty(ERRORS_PROPERTY, errorMap);
  		}
  
  		List<String> fieldErrors = errorMap.get(field);
  		if (fieldErrors == null) {
  			fieldErrors = new ArrayList<>();
  			errorMap.put(field, fieldErrors);
  		}
  		fieldErrors.addAll(errors);
  
  		addResultProperty(SUCCESS_PROPERTY, Boolean.FALSE);
  	}
docs: Adds multiple error messages to a specific field. Does not overwrite already
  existing errors.
  
  @param field the name of the field
  @param errors a collection of error messages