[avalanche] Improve invalid-signature error message accuracy

Summary:
This error relates to an invalid stake signature, so use that. This will make it easier to distiguish the error when we introduce proof signature.

Ref T1676.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1676

Differential Revision: https://reviews.bitcoinabc.org/D10101