[avalanche] Improve the delegateavalancheproof error messages

Summary:
This makes the error codes and messages more consistent with the other RPCs.

Ref T1635.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Maniphest Tasks: T1635

Differential Revision: https://reviews.bitcoinabc.org/D9779