Make the Redis connection more robust

When Redis resets the connection, the ECONNRESET error cause the nodejs server
to exit. Capture and log it so that the server lives on. It will reconnect, and
perhaps we'll learn that different errors require different handling after
enough logging.

index.js

@@ -15,6 +15,9 @@ if (process.env.REDIS_SECRET) {
   client.auth(process.env.REDIS_SECRET);
 }
 
+client.on('error', function (err) {
+  console.log('Redis client error: ' + err);
+});
 
 app.use(function* () {