Redirect to /unread when getting a 404 for an unread expired entry

Hosted miniflux don't keep read entries for long. If I leave my PC at
home on an unread entry, read everything pending at work and get back
to home later, when I click "next", I get a bare 404 error. I have to
go back to /unread myself. I think it would be more user friendly (but
maybe a bit suprising) to go directly to /unread in this case.