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/string.js

@@ -0,0 +1,11 @@
+#!/usr/bin/env node
+var argv = require('optimist')
+    .string('x', 'y')
+    .argv
+;
+console.dir([ argv.x, argv.y ]);
+
+/* Turns off numeric coercion:
+    ./node string.js -x 000123 -y 9876
+    [ '000123', '9876' ]
+*/