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/shim.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var getPolyfill = require('./polyfill');
+var define = require('define-properties');
+
+module.exports = function shimGetOwnPropertyDescriptors() {
+	var polyfill = getPolyfill();
+	define(
+		Object,
+		{ getOwnPropertyDescriptors: polyfill },
+		{ getOwnPropertyDescriptors: function () { return Object.getOwnPropertyDescriptors !== polyfill; } }
+	);
+	return polyfill;
+};