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/example/bool.js

@@ -0,0 +1,10 @@
+#!/usr/bin/env node
+var util = require('util');
+var argv = require('optimist').argv;
+
+if (argv.s) {
+    util.print(argv.fr ? 'Le chat dit: ' : 'The cat says: ');
+}
+console.log(
+    (argv.fr ? 'miaou' : 'meow') + (argv.p ? '.' : '')
+);