RPC/Blockchain: Clarify invalid-action error in scantxoutset

Partial backport of bitcoin#26508

Only the scantxoutset changes are applicable as Dash does not have
the scanblocks RPC function.

Co-authored-by: Luke Dashjr <[email protected]>

Author: Claude Code

src/rpc/blockchain.cpp

@@ -2584,7 +2584,7 @@ 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;
 },

test/functional/rpc_scantxoutset.py

@@ -128,8 +128,8 @@ def run_test(self):
         # Check that second arg is needed for start
         assert_raises_rpc_error(-1, "scanobjects argument is required for the start action", self.nodes[0].scantxoutset, "start")
 
-        # Check that invalid command give error
-        assert_raises_rpc_error(-8, "Invalid command", self.nodes[0].scantxoutset, "invalid_command")
+        # Check that invalid action gives error
+        assert_raises_rpc_error(-8, "Invalid action 'invalid_command'", self.nodes[0].scantxoutset, "invalid_command")
 
 
 if __name__ == "__main__":