log when _retrieveNextMessages's call to lokiRpc fails

js/modules/loki_message_api.js

@@ -399,6 +399,11 @@ class LokiMessageAPI {
       nodeData
     );
 
+    if (result === false) {
+      // make a note of it because of caller doesn't care...
+      log.warn(`loki_message:::_retrieveNextMessages - lokiRpc returned false to ${nodeData.ip}:${nodeData.port}`);
+    }
+
     return result.messages || [];
   }