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/object.getownpropertydescriptors/polyfill.js

@@ -0,0 +1,7 @@
+'use strict';
+
+var implementation = require('./implementation');
+
+module.exports = function getPolyfill() {
+	return typeof Object.getOwnPropertyDescriptors === 'function' ? Object.getOwnPropertyDescriptors : implementation;
+};