fix error code for delete folder (#42470) (#42519)

(cherry picked from commit e13e6a7bd7)

Co-authored-by: ying-jeanne <74549700+ying-jeanne@users.noreply.github.com>

pkg/services/libraryelements/database.go

@@ -692,9 +692,15 @@ func (l *LibraryElementService) deleteLibraryElementsInFolderUID(c context.Conte
 		if err != nil {
 			return err
 		}
+
+		if len(folderUIDs) == 0 {
+			return models.ErrFolderNotFound
+		}
+
 		if len(folderUIDs) != 1 {
 			return fmt.Errorf("found %d folders, while expecting at most one", len(folderUIDs))
 		}
+
 		folderID := folderUIDs[0].ID
 
 		if err := l.requirePermissionsOnFolder(c, signedInUser, folderID); err != nil {