HTTP response code

REST API always give `200 OK` even when failed to parse JSON.

client ajax request is like this

``` js
$http.post('<rest api addr>')
  .success(function(data) {
    // some statements for success.
  })
  .error(function(data) {
    // some satements for error.
  });
```

but this code only run success statements because the API server gives 200 OK
