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/util.promisify/index.js

@@ -0,0 +1,24 @@
+'use strict';
+
+var define = require('define-properties');
+var util = require('util');
+
+var implementation = require('./implementation');
+var getPolyfill = require('./polyfill');
+var polyfill = getPolyfill();
+var shim = require('./shim');
+
+/* eslint-disable no-unused-vars */
+var boundPromisify = function promisify(orig) {
+/* eslint-enable no-unused-vars */
+	return polyfill.apply(util, arguments);
+};
+define(boundPromisify, {
+	custom: polyfill.custom,
+	customPromisifyArgs: polyfill.customPromisifyArgs,
+	getPolyfill: getPolyfill,
+	implementation: implementation,
+	shim: shim
+});
+
+module.exports = boundPromisify;