3.15 regression: ListSerializer ValidationError nested structure silently changed

[peterthomassen]

#8863 changed the nested dict/list structure of ValidationError detail. I know this is the intention of #8863, but it does break code that parses exception details.

Note that much of that code is on the client side, which often cannot easily be comprehensively tested centrally (unlike backend-side parsing). This causes an unknown amount of misrendered error messages that lose their utility to human users. (And not only that; programmatic handling of structured errors of course breaks as well.)

This is a breaking change in a public-facing data structure and should be reverted.

[tomchristie]

Thanks, yes.
Let's treat this a duplicate of #9317.