Merge bitcoin/bitcoin#26508: RPC/Blockchain: Minor improvements for scanblocks & scantxoutset docs/errors

f986984 RPC/blockchain: scan{blocks,txoutset>: Further doc improvements (Luke Dashjr)
54b45e1 RPC/Blockchain: Clarify invalid-action error in scanblocks & scantxoutset (Luke Dashjr)

Pull request description:

  * Clarify invalid-action error in scanblocks & scantxoutset
  * Mention action=='start' only returns after scan completes (already in scantxoutset)
  * Document `relevant_blocks`

ACKs for top commit:
  kristapsk:
    utACK f986984
  aureleoules:
    ACK f986984
  MarnixCroes:
    ACK f986984

Tree-SHA512: a37c9cc8a9a2f59376e8d8ed7dbf5e140eb3fefb4b7c19a23fc8190f3aef060bda1f0d5d06dc81cd7dca9e871d65f6c8094bab6e8d42e0bcef0fc7ffd2342d09

Author: MacroFake

src/rpc/blockchain.cpp

@@ -2205,7 +2205,7 @@ static RPCHelpMan scantxoutset()
         result.pushKV("unspents", unspents);
         result.pushKV("total_amount", ValueFromAmount(total_in));
     } else {
-        throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid command");
+        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid action '%s'", request.params[0].get_str()));
     }
     return result;
 },
@@ -2254,12 +2254,13 @@ static RPCHelpMan scanblocks()
         },
         {
             scan_result_status_none,
-            RPCResult{"When action=='start'", RPCResult::Type::OBJ, "", "", {
+            RPCResult{"When action=='start'; only returns after scan completes", RPCResult::Type::OBJ, "", "", {
                 {RPCResult::Type::NUM, "from_height", "The height we started the scan from"},
                 {RPCResult::Type::NUM, "to_height", "The height we ended the scan at"},
-                {RPCResult::Type::ARR, "relevant_blocks", "", {{RPCResult::Type::STR_HEX, "blockhash", "A relevant blockhash"},}},
-                },
-            },
+                {RPCResult::Type::ARR, "relevant_blocks", "Blocks that may have matched a scanobject.", {
+                    {RPCResult::Type::STR_HEX, "blockhash", "A relevant blockhash"},
+                }},
+            }},
             RPCResult{"when action=='status' and a scan is currently in progress", RPCResult::Type::OBJ, "", "", {
                     {RPCResult::Type::NUM, "progress", "Approximate percent complete"},
                     {RPCResult::Type::NUM, "current_height", "Height of the block currently being scanned"},
@@ -2402,7 +2403,7 @@ static RPCHelpMan scanblocks()
         ret.pushKV("relevant_blocks", blocks);
     }
     else {
-        throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid command");
+        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid action '%s'", request.params[0].get_str()));
     }
     return ret;
 },

test/functional/rpc_scanblocks.py

@@ -122,7 +122,7 @@ def run_test(self):
         assert_equal(node.scanblocks("abort"), False)
 
         # test invalid command
-        assert_raises_rpc_error(-8, "Invalid command", node.scanblocks, "foobar")
+        assert_raises_rpc_error(-8, "Invalid action 'foobar'", node.scanblocks, "foobar")
 
 
 if __name__ == '__main__':