Create a 500 error page function in html-formatter for general errors

When the Service Worker routes need to error (e.g., in a `catch`), we need some logic to create a proper error page. For example, with `err.code === 'ENOENT'` we should format a 404 page and return that. With other types of errors we might do some other page to indicate a problem, and in the general case, we need a custom 500 page, which might also print some info to help diagnose.