def err_response(reqid, code, msg, data=None):
    """
    Formats a JSON-RPC error as a dict with keys: 'jsonrpc', 'id', 'error'
    """
    err = {'code': code, 'message': msg}
    if data:
        err['data'] = data
    return {'jsonrpc': '2.0', 'id': reqid, 'error': err}
