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/fast-json-stable-stringify/test/to-json.js

@@ -0,0 +1,22 @@
+'use strict';
+
+var test = require('tape');
+var stringify = require('../');
+
+test('toJSON function', function (t) {
+    t.plan(1);
+    var obj = { one: 1, two: 2, toJSON: function() { return { one: 1 }; } };
+    t.equal(stringify(obj), '{"one":1}' );
+});
+
+test('toJSON returns string', function (t) {
+    t.plan(1);
+    var obj = { one: 1, two: 2, toJSON: function() { return 'one'; } };
+    t.equal(stringify(obj), '"one"');
+});
+
+test('toJSON returns array', function (t) {
+    t.plan(1);
+    var obj = { one: 1, two: 2, toJSON: function() { return ['one']; } };
+    t.equal(stringify(obj), '["one"]');
+});