[avalanche] Let isfinaltransaction raise an error when the transaction is not found

Summary:
This makes it easier to guide the user through the reason why the transaction is marked as non final, and hints on how to let the node find the transaction. The error messages are shamelessly copied from getrawtransaction which has the same behavior.

Depends on D12002.

Test Plan:

./test/functional/test_runner.py abc_rpc_isfinal

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12001