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/polyfill.js

@@ -0,0 +1,11 @@
+'use strict';
+
+var util = require('util');
+var implementation = require('./implementation');
+
+module.exports = function getPolyfill() {
+	if (typeof util.promisify === 'function') {
+		return util.promisify;
+	}
+	return implementation;
+};