cave/feed: report url_host() errors

cave/src/feed.rs

@@ -5,6 +5,7 @@ use eventsource_stream::Eventsource;
 
 pub fn url_host(url: &str) -> Option<String> {
     reqwest::Url::parse(url)
+        .map_err(|e| tracing::warn!("Cannot parse url {:?}: {}", url, e))
         .ok()
         .and_then(|url| url.domain()
                   .map(|host| host.to_lowercase())