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/pn/punycode.js

@@ -0,0 +1,3 @@
+var punycode = {};
+try { punycode = require("punycode"); } catch (e) { }
+module.exports = punycode;