By the way, there's a subtle change here that's important to mention. This used to be testing the "rollback", where the form saves but something goes wrong after the initial form save and before the successful_form_received signal and the form gets re-saved as an `XFormError`. Now it's testing that if something goes wrong _during_ a signal, the form gets tagged with a problem; the doc_type does not change which isn't explicitly tested here.

(Oddly, `couchforms/all_submissions_by_domain` lumps `XFormInstance`s with a problem field together with `XFormError`s, hence not needing to change the view call, but needing to remove the `classes` restriciton