code: def format_error(invalid, doc_type):
      
      # using string for checking is probably not ideal,
      # but voluptuous does not have specific sub error
      # types for these errors
      if invalid.error_message == 'extra keys not allowed':
          msg = "Key '{}' is not allowed".format(invalid.path[0])
      elif invalid.error_message == 'required key not provided':
          msg = "{} '{}' is missing".format(doc_type, invalid.path[0])
      else:
          msg = invalid.message
      return {'message': msg, 'field': str(invalid.path[0])}
docs: format the error message using the voluptuous.Invalid object
      :param invalid: voluptuous.Invalid instance
      :param doc_type: type of the object, e.g. Organisation, User