[avalanche] Improve error messages from the addavalanchenode RPC

Summary:
Rather than returning false, it is possible to raise an error with a meaningful message.

Depends on D9792 and D9794.

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/D9793