500 Server Error when trying to create a new resource for a package that doesn't exist

Visit `http://127.0.0.1:5000/dataset/new_resource/package-name-that-does-not-exist`, get a 500. Uncaught exception:

```
File '/home/seanh/Projects/ckan/ckan/ckan/logic/action/get.py', line 734 in package_show
  raise NotFound
NotFound: 
```