DNS errors don't recover

[janw]

Extracting from #129, quoting @SpangleLabs:

A minor issue, but something I noticed as I seem to have a lot of broken old feeds. I noticed that if the feed url is a 404 that the archiver logs the error and moves on to the next feed, which is good! But if the feed gives a DNS error, it seems the archiver throws an error and halts.
An example of the former: https://www.ministryoftesting.com/feed/podcast
An example of the latter: https://feeds.sounder.fm/1188/rss.xml
I would drop the stack traces here, but they're very long.