Bitcoin Core 28+ replies to a JSON-RPC 2.0 request with a strict 2.0
response: a success has a 'result' key but no 'error' key. The v1.11 code
did 'if result["error"] is not None' and crashed with KeyError: 'error'.
The getters are already guarded on dev with 'if "error" in result and
...'; add regression tests for get_network_info / get_blockchain_info so
that guard can't be dropped again (verified they fail against the old
unguarded form).
Fixes#277
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
