Tasks with artifacts cannot be edited in the Django admin

That took me a long time to figure out…

1. Open any Ponos task in the Django admin that has some artifacts listed in the Artifacts inline at the bottom of the page.
2. Change nothing, just click any of the Save buttons.
3. A "Please correct the error below:" message appears, with no error being displayed at all on the page. The view's template gets a {"errors": [["This field is required."]]} in its context, which it cannot display.
4. Cry.

It turns out Django does not understand that artifacts cannot be edited, only deleted.