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/has-value/index.js

@@ -0,0 +1,16 @@
+/*!
+ * has-value <https://github.com/jonschlinkert/has-value>
+ *
+ * Copyright (c) 2014-2017, Jon Schlinkert.
+ * Licensed under the MIT License.
+ */
+
+'use strict';
+
+var isObject = require('isobject');
+var hasValues = require('has-values');
+var get = require('get-value');
+
+module.exports = function(val, prop) {
+  return hasValues(isObject(val) && prop ? get(val, prop) : val);
+};