code: def host_errors(self, hostname, details):
          
          if not hostname or not isinstance(hostname, basestring):
              return "hostname_invalid"
          if not isinstance(details, list):
              return "host_details_not_list"
          if len(details) != 3:
              return "host_details_wrong_length"
          if details[0] not in self.balancer.action_mapping:
              return "host_action_invalid:%s" % details[0]
          if not isinstance(details[1], dict):
              return "host_kwargs_not_dict"
          if not isinstance(details[2], bool):
              return "host_match_subdomains_not_bool"
          return None
docs: Validates the format of a host entry
          Returns an error string, or None if it is valid.