[BUU] Variant updates ignored when error on bulk updating products

[dacook]

Description

Raised in https://github.com/openfoodfoundation/openfoodnetwork/pull/11750:

When product and associated variant are updated, but there are errors:
Neither should be saved, summary should show, and unsaved values retained with red/orange border. But these fail due to #11748

1. product has error, variant has error. 🆇 Variant values are lost
2. product has error, variant has valid changes. 🆇 Variant values are lost
3. product has valid changes, variant has error. 🆇 Product changes got saved, but shouldn't have been until variant error was resolved

Expected Behavior

Actual Behaviour

Steps to Reproduce

Animated Gif/Screenshot

Workaround

Severity

Your Environment

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• Operating System and version (desktop or mobile):

Possible Fix

See https://github.com/openfoodfoundation/openfoodnetwork/issues/11748