playground: Improve client error messages.

Addressed nit by @sadovsky.
diff --git a/client/playground/client/embedded.js b/client/playground/client/embedded.js
index abda139..838fa8b 100644
--- a/client/playground/client/embedded.js
+++ b/client/playground/client/embedded.js
@@ -231,7 +231,7 @@
   }));
 
   req.on('close', ifRunActive(function() {
-    // partialLine should be empty when connection is closed.
+    // Sanity check: partialLine should be empty when connection is closed.
     partialLine = partialLine.trim();
     if (partialLine) {
       console.error('Connection closed without newline after: ' + partialLine);