You can now redirect all 404 URLs of your store to new URLs using 404 URL Redirects Manager Extension. It not only allows you to set new working URLs for all 404 URLs of your website, but it can also fetch list of all crawl errors of your website. You simply need to configure your webmaster tools account details while configuring this extension and then you can download CSV file for crawl errors of your website. You can manually set new URL against current 404 URL of your website and can also import CSV file for bulk import of Old URLs and New URLs.