Merge bitcoin/bitcoin#23706: rpc: getblockfrompeer followups

923312fbf6 rpc: use peer_id, block_hash for FetchBlock (Sjors Provoost)
34d5399211 rpc: more detailed errors for getblockfrompeer (Sjors Provoost)
60243cac72 rpc: turn already downloaded into error in getblockfrompeer (Sjors Provoost)
809d66bb65 rpc: clarify getblockfrompeer behavior when called multiple times (Sjors Provoost)
0e3d7c5ee1 refactor: drop redundant hash argument from FetchBlock (Sjors Provoost)
8d1a3e6498 rpc: allow empty JSON object result (Sjors Provoost)
bfbf91d0b2 test: fancier Python for getblockfrompeer (Sjors Provoost)

Pull request description:

  Followups from #20295.

ACKs for top commit:
  jonatack:
    ACK 923312fbf6 📦
  fjahr:
    tested ACK 923312fbf6

Tree-SHA512: da9eca76e302e249409c9d7f0d16cca668ed981e2ab6ca2d1743dad0d830b94b1bc5ffb9028a00764b863201945c273cc8f4409a4c9ca3817830007dffa2bc20

src/net_processing.h

@@ -45,12 +45,11 @@ public:
     /**
      * Attempt to manually fetch block from a given peer. We must already have the header.
      *
-     * @param[in]  id       The peer id
-     * @param[in]  hash     The block hash
-     * @param[in]  pindex   The blockindex
-     * @returns             Whether a request was successfully made
+     * @param[in]  peer_id      The peer id
+     * @param[in]  block_index  The blockindex
+     * @returns std::nullopt if a request was successfully made, otherwise an error message
      */
-    virtual bool FetchBlock(NodeId id, const uint256& hash, const CBlockIndex& pindex) = 0;
+    virtual std::optional<std::string> FetchBlock(NodeId peer_id, const CBlockIndex& block_index) = 0;
 
     /** Begin running background tasks, should only be called once */
     virtual void StartScheduledTasks(CScheduler& scheduler) = 0;