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/shim.js

@@ -0,0 +1,13 @@
+'use strict';
+
+var util = require('util');
+var getPolyfill = require('./polyfill');
+
+module.exports = function shimUtilPromisify() {
+	var polyfill = getPolyfill();
+	if (polyfill !== util.promisify) {
+		util.promisify = polyfill;
+		Object.defineProperty(util, 'promisify', { value: polyfill });
+	}
+	return polyfill;
+};