ImportError in Django admin when viewing result list that includes a deleted task

[tomkins]

Fairly easy one to recreate:

• Create a task
• Run the task with the database backend (so it populates DBTaskResult)
• Comment out or delete the task
• Go to the admin: http://127.0.0.1:8000/admin/django_tasks_database/dbtaskresult/ - see a 500

The detail view is not impacted - it's just the list view with the following exception:

ImportError at /admin/django_tasks_database/dbtaskresult/
Module "taskrunner.tasks" does not define a "demo_fail" attribute/class

I'm assuming that it's perfectly fine/safe to keep results for tasks that no longer exist.

Will follow up with a PR shortly.