[avalanche] Make the deserialization error code consistent between RPCs

Summary:
RPC_DESERIALIZATION_ERROR is a better code than
RPC_INVALID_PARAMETER when an hex proof cannot be deserialized.

Ref T1611.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1611

Differential Revision: https://reviews.bitcoinabc.org/D9649