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/optimist/test/_/argv.js

@@ -0,0 +1,2 @@
+#!/usr/bin/env node
+console.log(JSON.stringify(process.argv));

node_modules/optimist/test/_/bin.js

@@ -0,0 +1,3 @@
+#!/usr/bin/env node
+var argv = require('../../index').argv
+console.log(JSON.stringify(argv._));