correctly serialize an error log

lib/hk-util.js

@@ -621,7 +621,10 @@ const handleGetHistory = function (Env, Server, seq, userId, parsed) {
             Server.send(userId, [0, HISTORY_KEEPER_ID, 'MSG', userId, JSON.stringify(msg)], readMore);
         }, (err) => {
             if (err && err.code !== 'ENOENT') {
-                if (err.message !== 'EINVAL') { Log.error("HK_GET_HISTORY", err); }
+                if (err.message !== 'EINVAL') { Log.error("HK_GET_HISTORY", {
+                    err: err && err.message,
+                    stack: err && err.stack,
+                }); }
                 const parsedMsg = {error:err.message, channel: channelName, txid: txid};
                 Server.send(userId, [0, HISTORY_KEEPER_ID, 'MSG', userId, JSON.stringify(parsedMsg)]);
                 return;