Websites go down from time to time. Fortunately, if you just need to access a page you've visited previously to grab some details, you don't have to worry about getting a 404, thanks to Chrome's cache, which Google has made more accessible than ever.
Whenever you visit a website, Chrome (and any browser) will cache text, images, scripts and other data required to render the page. So for static content, you don't need to revisit a page -- it's already stored on your machine.
The only problem is accessing this saved data in an easy way.
Rather than use an extension, Chrome supports this functionality already. It's not enabled by default, so you'll have to hit up the
chrome://flags page to switch it on.
Do an in-page search for "Show Saved Copy Button", and change it from "Disabled" to "Enabled: Primary".
If you want to know the difference between the primary and secondary options, here's how they're described on the flags page:
When a page fails to load, if a stale copy of the page exists in the browser cache, a button will be presented to allow the user to load that stale copy. The primary enabling choice puts the button in the most salient position on the error page; the secondary enabling choice puts it secondary to the reload button.
Once active (and you've restarted the browser), the usual Chrome error page will have a new button labelled "Show Saved Copy". If a copy is available, you'll see that instead.