Format an error message if server selection fails.

        Hold the lock when calling this.

def _error_message(self, selector):
        """Format an error message if server selection fails.

        Hold the lock when calling this.
        """
        is_replica_set = self._description.topology_type in (
            TOPOLOGY_TYPE.ReplicaSetWithPrimary,
            TOPOLOGY_TYPE.ReplicaSetNoPrimary)

        if is_replica_set:
            server_plural = 'replica set members'
        elif self._description.topology_type == TOPOLOGY_TYPE.Sharded:
            server_plural = 'mongoses'
        else:
            server_plural = 'servers'

        if self._description.known_servers:
            # We've connected, but no servers match the selector.
            if selector is writable_server_selector:
                if is_replica_set:
                    return 'No primary available for writes'
                else:
                    return 'No %s available for writes' % server_plural
            else:
                return 'No %s match selector "%s"' % (server_plural, selector)
        else:
            addresses = list(self._description.server_descriptions())
            servers = list(self._description.server_descriptions().values())
            if not servers:
                if is_replica_set:
                    # We removed all servers because of the wrong setName?
                    return 'No %s available for replica set name "%s"' % (
                        server_plural, self._settings.replica_set_name)
                else:
                    return 'No %s available' % server_plural

            # 1 or more servers, all Unknown. Are they unknown for one reason?
            error = servers[0].error
            same = all(server.error == error for server in servers[1:])
            if same:
                if error is None:
                    # We're still discovering.
                    return 'No %s found yet' % server_plural

                if (is_replica_set and not
                        set(addresses).intersection(self._seed_addresses)):
                    # We replaced our seeds with new hosts but can't reach any.
                    return (
                        'Could not reach any servers in %s. Replica set is'
                        ' configured with internal hostnames or IPs?' %
                        addresses)

                return str(error)
            else:
                return ','.join(str(server.error) for server in servers
                                if server.error)