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/atob/bin/atob.js

@@ -0,0 +1,6 @@
+#!/usr/bin/env node
+'use strict';
+
+var atob = require('../node-atob');
+var str = process.argv[2];
+console.log(atob(str));