当我不想处理所有404错误时，如何在Scrapy中返回404错误？

from scrapy.spidermiddlewares.httperror import HttpErrorMiddleware
from scrapy.exceptions import IgnoreRequest

class CustomErrorMiddleware(HttpErrorMiddleware):
    def process_spider_exception(self, response, exception, spider):
        if isinstance(exception, HttpError) and response.status == 404:
            # 处理404错误的逻辑
            # 这里可以自定义你想要的操作，比如记录日志、发送通知等
            # 如果你想忽略该请求，可以抛出IgnoreRequest异常
            raise IgnoreRequest
        return None

SPIDER_MIDDLEWARES = {
    'your_project_name.middlewares.CustomErrorMiddleware': 543,
}