Don't show unnecessary error message to end users for DeleteBranchAfterMerge (#35937) (#35941)

Backport #35937 by wxiaoguang

Co-authored-by: wxiaoguang <[email protected]>

Author: GiteaBot

routers/web/repo/pull.go

@@ -1208,7 +1208,11 @@ func MergePullRequest(ctx *context.Context) {
 func deleteBranchAfterMergeAndFlashMessage(ctx *context.Context, prID int64) {
 	var fullBranchName string
 	err := repo_service.DeleteBranchAfterMerge(ctx, ctx.Doer, prID, &fullBranchName)
-	if errTr := util.ErrorAsTranslatable(err); errTr != nil {
+	if errors.Is(err, util.ErrPermissionDenied) || errors.Is(err, util.ErrNotExist) {
+		// no need to show error to end users if no permission or branch not exist
+		log.Debug("DeleteBranchAfterMerge (ignore unnecessary error): %v", err)
+		return
+	} else if errTr := util.ErrorAsTranslatable(err); errTr != nil {
 		ctx.Flash.Error(errTr.Translate(ctx.Locale))
 		return
 	} else if err == nil {