Get the balance of a particular token
        Returns {'balance': ...}

def GET_account_balance(self, path_info, account_addr, token_type):
        """
        Get the balance of a particular token
        Returns {'balance': ...}
        """
        if not check_account_address(account_addr):
            return self._reply_json({'error': 'Invalid address'}, status_code=400)

        if not check_token_type(token_type):
            return self._reply_json({'error': 'Invalid token type'}, status_code=400)

        blockstackd_url = get_blockstackd_url()
        res = blockstackd_client.get_account_balance(account_addr, token_type, hostport=blockstackd_url)
        if json_is_error(res):
            log.error("Failed to get account balance for {} {}: {}".format(account_addr, token_type, res['error']))
            return self._reply_json({'error': 'Failed to get balance of {} for {}: {}'.format(token_type, account_addr, res['error'])}, status_code=res.get('http_status', 500))

        self._reply_json({'balance': str(res)})     # NOTE: use a string, since this can be too big for js clients to parse
        return