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/example/key_cmp.js

@@ -0,0 +1,7 @@
+var stringify = require('../');
+
+var obj = { c: 8, b: [{z:6,y:5,x:4},7], a: 3 };
+var s = stringify(obj, function (a, b) {
+    return a.key < b.key ? 1 : -1;
+});
+console.log(s);