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/xup.js

@@ -0,0 +1,10 @@
+#!/usr/bin/env node
+var argv = require('optimist').argv;
+
+if (argv.rif - 5 * argv.xup > 7.138) {
+    console.log('Buy more riffiwobbles');
+}
+else {
+    console.log('Sell the xupptumblers');
+}
+