rpc: turn already downloaded into error in getblockfrompeer

Summary:

rpc: allow empty JSON object result

https://github.com/bitcoin/bitcoin/pull/23706/commits/8d1a3e6498de6087501969a9d243b0697ca3fe97

rpc: turn already downloaded into error in getblockfrompeer

https://github.com/bitcoin/bitcoin/pull/23706/commits/60243cac7286e4c4bdda7094bef4cf6d1564b583

This is a partial backport of core#23706
Depends on D12717

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12719