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/node-atob.js

@@ -0,0 +1,7 @@
+"use strict";
+
+function atob(str) {
+  return Buffer.from(str, 'base64').toString('binary');
+}
+
+module.exports = atob.atob = atob;