Show a feed that answers with nothing, having never posted, as failing (#119)

daily-quests and hots-dots had 0 items and no error, so their subscribers saw shows that had not
posted yet rather than feeds that are gone. Both domains have lapsed, and this network's DNS
filter answers for a parked domain with its block page, over http a 200 with an empty body.
scan_one took a body of nothing but whitespace for "nothing yet" and recorded no error.

It still does for a feed that has stored items, which is what the Antarctic Survey's empty 202
means, but a feed that has never stored one now fails with "the feed's address answered with
nothing at all", which the feed's page explains as an address that answers with nothing, the
site maybe gone. The next real read clears it. A test serves both from a socket of its own.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/feed.rs

@@ -190,6 +190,9 @@ pub fn explain_failure(msg: &str) -> Option<Failure> {
     if low.contains("http 401") || low.contains("http 403") {
         return Some(Failure { reason: "The site refuses ipx's requests.", new_url: None });
     }
+    if low.contains("answered with nothing") {
+        return Some(Failure { reason: "This address answers with nothing; the site may be gone.", new_url: None });
+    }
     if low.contains("http 402") {
         return Some(Failure { reason: "The feed now needs a paid plan.", new_url: None });
     }
@@ -1310,6 +1313,10 @@ mod tests {
         assert_eq!(explain_failure("HTTP 401 Unauthorized").unwrap().reason, "The site refuses ipx's requests.");
         assert_eq!(explain_failure("HTTP 403 Forbidden").unwrap().reason, "The site refuses ipx's requests.");
         assert_eq!(explain_failure("HTTP 402 Payment Required").unwrap().reason, "The feed now needs a paid plan.");
+        assert_eq!(
+            explain_failure("the feed's address answered with nothing at all").unwrap().reason,
+            "This address answers with nothing; the site may be gone."
+        );
         let dns = explain_failure("connecting: dns error: failed to lookup address information").unwrap();
         assert_eq!(dns.reason, "This address no longer resolves; the site is gone.");
         let moved = explain_failure("got a web page, not a feed; it links https://x/feed as its feed").unwrap();