title: django-tastypie to have JSON response
body: <p>What I meant exactly was, I would like to have JSON response when I modify the obj_create(). I've implemented the UserSignUpResource(ModelResource) and inside the obj_create(), I did some validation and when it fails, I raise BadRequest(). However, this doesn't throw out JSON. It throws out String instead.</p>
  
  <p>Any idea if I can make it throw out {'error': 184, 'message': 'This username already exists'} format? Or am I not suppose to modify obj_create()? Or what should I do instead?</p>
  
  <p>Many help, thanks.</p>
  
  <p>Cheers,
  Mickey </p>