[avalanche] Make the error code consistent in addavalanchenode

Summary:
Deserialization errors should return a deserialization error.

Depends on D9786.

Ref T1635.

Test Plan:

./test/functional/test_runner.py abc_rpc_addavalanchenode

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1635

Differential Revision: https://reviews.bitcoinabc.org/D9787