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/src/index.js

@@ -0,0 +1,5 @@
+'use strict';
+
+var bind = require('function-bind');
+
+module.exports = bind.call(Function.call, Object.prototype.hasOwnProperty);