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/boolean_single.js

@@ -0,0 +1,7 @@
+#!/usr/bin/env node
+var argv = require('optimist')
+    .boolean('v')
+    .argv
+;
+console.dir(argv.v);
+console.dir(argv._);