Handled LookupError in RelatedLookup view

Calling the `RelatedLookup` with wrong `app_label` and `model_name` parameters curently raises a 500. 
Example: `/grappelli/lookup/related/?object_id=&app_label=false&model_name=false`

The pull request handles the error so that the view returns `[{"value": null, "label": ""}]` in this case.