Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/ws/index.js

@@ -0,0 +1,9 @@
+'use strict';
+
+const WebSocket = require('./lib/websocket');
+
+WebSocket.Server = require('./lib/websocket-server');
+WebSocket.Receiver = require('./lib/receiver');
+WebSocket.Sender = require('./lib/sender');
+
+module.exports = WebSocket;