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/qs/lib/index.js

@@ -0,0 +1,11 @@
+'use strict';
+
+var stringify = require('./stringify');
+var parse = require('./parse');
+var formats = require('./formats');
+
+module.exports = {
+    formats: formats,
+    parse: parse,
+    stringify: stringify
+};