Fixes #12402 - When bulk-importing related objects, add the errors to the base form instead of the model_form

[kkthxbye-code]

Fixes: #12402

The errors of related models are currently added to the model_form. If there's any errors in a related object form, an AbortTransaction is thrown. The errors saved on the model form are never replicated to the base form and thus are never shown on the form.

I'm really unsure of the fix in this PR, but it seems to work. The code is pretty complex imo, a lot of different forms being passed around, so not sure if there's a better way. Feel free to just close this PR and fix it another way, just wanted to have it written down.

[jeremystretch]

I think the root issue here is that the related object forms are not validated until after we've begun to save the parent object. We'll need to unwind some of the bulk logic to correct this.

[kkthxbye-code]

the related object forms are not validated until after we've begun to save the parent object

I'm assuming this is because the clean methods do validation on the reference between the related object and the parent, so we need the pk of the parent before we can validate the related object?

Regardless, feel free to close this PR and un-assign me, it was mostly just a way to get the ball rolling 👍

[jeremystretch]

Understood, and thanks for taking it on.