Improve error message for when a UA is only usable after a future NU.

Co-authored-by: Daira Hopwood <daira@jacaranda.org>

src/wallet/rpcwallet.cpp

@@ -4915,7 +4915,7 @@ UniValue z_sendmany(const UniValue& params, bool fHelp)
             } else {
                 throw JSONRPCError(
                     RPC_INVALID_PARAMETER,
-                    "Unified address contained no recognized receiver types.");
+                    "Unified address contained only receiver types that are unrecognized or for which the required consensus feature is not yet active.");
             }
         }