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/minimist/test/parse_modified.js

@@ -0,0 +1,9 @@
+var parse = require('../');
+var test = require('tape');
+
+test('parse with modifier functions' , function (t) {
+    t.plan(1);
+    
+    var argv = parse([ '-b', '123' ], { boolean: 'b' });
+    t.deepEqual(argv, { b: true, _: [123] });
+});