3.15 regression: ListSerializer ValidationErrors silently changed return type

[peterthomassen]

• ListSerializer.errors has type ReturnList.
• When raising, feat: Add some changes to ValidationError to support django style vad… #8863 silently casts this into a list.
• As a result, the exceptions .detail attribute is no longer of type ReturnList.
• As a result, the .detail.serializer attribute is no longer available. That attribute is the whole point of the existence of ReturnList.

This is a breaking change in a public API and should be reverted.