fix(server): use the correct err for internal server log (#563)

### Motivation

use the correct error for the log.

server/internal_rpc_server.go

@@ -172,14 +172,14 @@ func (s *internalRpcServer) BecomeLeader(c context.Context, req *proto.BecomeLea
 		return nil, err
 	}
 
-	res, err2 := leader.BecomeLeader(c, req)
-	if err2 != nil {
+	res, err := leader.BecomeLeader(c, req)
+	if err != nil {
 		log.Warn(
 			"BecomeLeader failed",
 			slog.Any("error", err),
 		)
 	}
-	return res, err2
+	return res, err
 }
 
 func (s *internalRpcServer) AddFollower(c context.Context, req *proto.AddFollowerRequest) (*proto.AddFollowerResponse, error) {
@@ -199,14 +199,14 @@ func (s *internalRpcServer) AddFollower(c context.Context, req *proto.AddFollowe
 		return nil, err
 	}
 
-	res, err2 := leader.AddFollower(req)
-	if err2 != nil {
+	res, err := leader.AddFollower(req)
+	if err != nil {
 		log.Warn(
 			"AddFollower failed",
 			slog.Any("error", err),
 		)
 	}
-	return res, err2
+	return res, err
 }
 
 func (s *internalRpcServer) Truncate(c context.Context, req *proto.TruncateRequest) (*proto.TruncateResponse, error) {
@@ -226,12 +226,14 @@ func (s *internalRpcServer) Truncate(c context.Context, req *proto.TruncateReque
 		return nil, err
 	}
 
-	res, err2 := follower.Truncate(req)
-	log.Warn(
-		"Truncate failed",
-		slog.Any("error", err),
-	)
-	return res, err2
+	res, err := follower.Truncate(req)
+	if err != nil {
+		log.Warn(
+			"Truncate failed",
+			slog.Any("error", err),
+		)
+	}
+	return res, err
 }
 
 func (s *internalRpcServer) Replicate(srv proto.OxiaLogReplication_ReplicateServer) error {
@@ -274,14 +276,14 @@ func (s *internalRpcServer) Replicate(srv proto.OxiaLogReplication_ReplicateServ
 		return err
 	}
 
-	err2 := follower.Replicate(srv)
-	if err2 != nil && !errors.Is(err2, io.EOF) {
+	err = follower.Replicate(srv)
+	if err != nil && !errors.Is(err, io.EOF) {
 		log.Warn(
 			"Replicate failed",
 			slog.Any("error", err),
 		)
 	}
-	return err2
+	return err
 }
 
 func (s *internalRpcServer) SendSnapshot(srv proto.OxiaLogReplication_SendSnapshotServer) error {
@@ -326,8 +328,8 @@ func (s *internalRpcServer) SendSnapshot(srv proto.OxiaLogReplication_SendSnapsh
 		return err
 	}
 
-	err2 := follower.SendSnapshot(srv)
-	if err2 != nil {
+	err = follower.SendSnapshot(srv)
+	if err != nil {
 		s.log.Warn(
 			"SendSnapshot failed",
 			slog.Any("error", err),
@@ -336,7 +338,7 @@ func (s *internalRpcServer) SendSnapshot(srv proto.OxiaLogReplication_SendSnapsh
 			slog.String("peer", common.GetPeer(srv.Context())),
 		)
 	}
-	return err2
+	return err
 }
 
 func (s *internalRpcServer) GetStatus(_ context.Context, req *proto.GetStatusRequest) (*proto.GetStatusResponse, error) {