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/ajv/scripts/info

@@ -0,0 +1,10 @@
+#!/usr/bin/env node
+
+'use strict';
+
+var fs = require('fs');
+var name = process.argv[2] || '.';
+var property = process.argv[3] || 'version';
+if (name != '.') name = 'node_modules/' + name;
+var json = JSON.parse(fs.readFileSync(name + '/package.json', 'utf8'));
+console.log(json[property]);