multi: prepare for new non-constant string rule

In preparation for the next commit which bumps golang to a newer
version, we want to make some code changes that would otherwise render
some log-related calls problematic. With go1.24 a new govet rule was
added that disallows non-constant strings (i.e including a tag like
"%s") in calls to printf. See more in the related issue
https://github.com/golang/go/issues/60529.

rpcmiddleware/proto.go

@@ -76,8 +76,12 @@ func RPCErrString(req *lnrpc.RPCMiddlewareRequest, format string,
 		},
 	}
 
+	if len(args) > 0 {
+		format = fmt.Sprintf(format, args...)
+	}
+
 	if format != "" {
-		feedback.Error = fmt.Sprintf(format, args...)
+		feedback.Error = format
 	}
 
 	return resp, nil