code: public FormInputValidator fileExists (VisValidatableTextField field, FileHandle relativeTo, String errorMsg) {
  		FileExistsValidator validator = new FileExistsValidator(relativeTo.file(), errorMsg);
  		field.addValidator(validator);
  		add(field);
  		return validator;
  
  	}
docs: Validates if relative path entered in text field points to an existing file.
  @param relativeTo path of this file is used to create absolute path from entered in field (see {@link FileExistsValidator}).