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/test/whitespace.js

@@ -0,0 +1,8 @@
+var optimist = require('../');
+var test = require('tap').test;
+
+test('whitespace should be whitespace' , function (t) {
+    t.plan(1);
+    var x = optimist.parse([ '-x', '\t' ]).x;
+    t.equal(x, '\t');
+});