Add noindex meta tag to 404.html

Since nobody wants search engines to index a 404 page, I think it's a good idea to add a `noindex` meta tag to `404.html`. This meta tag will ensure that search engines aren't indexing broken links.

Reference: [https://support.google.com/webmasters/answer/93710?hl=en](https://support.google.com/webmasters/answer/93710?hl=en)