docs: def add_errors(self, property_name, errors):
          """
          Add one or several errors to properties.
          :param property_name: str, property name
          :param errors: list or Error, error object(s)
          :return: shiftschema.result.Result
          """
          if type(errors) is not list:
              errors = [errors]
          for error in errors:
              if not isinstance(error, Error):
                  err = 'Error must be of type {}'
                  raise x.InvalidErrorType(err.format(Error))
  
          if property_name in self.errors:
              self.errors[property_name].extend(errors)
          else:
              self.errors[property_name] = errors
  
          return self