[avalanche] Consistently check ProofBuilder::addUTXO return value

Summary:
And use a [[nodiscard]] to demonstrate it. This ensure we are testing what we expect and not silently discarding duplicated stakes.

Ref T1676.
Depends on D10077 and D10078.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1676

Differential Revision: https://reviews.bitcoinabc.org/D10079