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-extendable/index.js

@@ -0,0 +1,13 @@
+/*!
+ * is-extendable <https://github.com/jonschlinkert/is-extendable>
+ *
+ * Copyright (c) 2015, Jon Schlinkert.
+ * Licensed under the MIT License.
+ */
+
+'use strict';
+
+module.exports = function isExtendable(val) {
+  return typeof val !== 'undefined' && val !== null
+    && (typeof val === 'object' || typeof val === 'function');
+};