Allow capturing uniqueness error

What does this MR do and why?

Uniqueness errors during value stream update is not handled, resulting in 500 error.

This is because Rails, when persisting an array of association records, would do a mass validation first, then persist all of them without validation. Uniqueness error can only surface during the persisting phase, as a database exception. This MR rescues such error and adds it to the ActiveModel errors, for API to serialize as response properly.

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Related to #415404 (closed)