refactor: entity not found exceptions handler

repositories/board.py

@@ -7,6 +7,7 @@ from schemas.board import UpdateBoardSchema, CreateBoardSchema
 
 class BoardRepository(RepCrudMixin[Board, CreateBoardSchema, UpdateBoardSchema]):
     entity_class = Board
+    entity_not_found_msg = "Доска не найдена"
 
     def _process_get_all_stmt_with_args(self, stmt: Select, *args) -> Select:
         project_id = args[0]