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/is-descriptor/index.js

@@ -0,0 +1,22 @@
+/*!
+ * is-descriptor <https://github.com/jonschlinkert/is-descriptor>
+ *
+ * Copyright (c) 2015-2017, Jon Schlinkert.
+ * Released under the MIT License.
+ */
+
+'use strict';
+
+var typeOf = require('kind-of');
+var isAccessor = require('is-accessor-descriptor');
+var isData = require('is-data-descriptor');
+
+module.exports = function isDescriptor(obj, key) {
+  if (typeOf(obj) !== 'object') {
+    return false;
+  }
+  if ('get' in obj) {
+    return isAccessor(obj, key);
+  }
+  return isData(obj, key);
+};