[avalanche] Move the vote eror codes to its own enum class

Summary:
And remove the BLOCK_ prefix. This will be used for writing tests for conflicting proof votes as well.

Ref T1854.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_voting

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10245