Stop reading dates at the first sign the server is unreachable

A window of cells whose thumbnails were cached but whose dates were not
sent a header read per cell, and offline each one was three attempts at
a 15 s connect timeout: `is_transient` counts a network error as worth
retrying, and `read_metadata_only` returned a bare bool that could not
say why a read failed. So the grid sat on "reading N dates" for minutes
against a server that was not there, and no banner went up, because
nothing in that loop ever reported the connection.

`read_metadata_only` now returns a `DateRead`: reached, failed, or
offline. An offline error is returned on the first attempt rather than
retried — a dead server answers the second exactly as the first — while
a 423 lock is still retried, which is what the retry was for. The grid's
worker stops on it and sends `Offline`, as its fetch loop already did,
so the banner goes up and the bar stops. The sweep's lanes stop on it
too, one timeout each rather than one per image.

ui/dr-ui/src/library/thumbnails_gen.rs

@@ -9,7 +9,7 @@ use dr_thumbs::ThumbStore;
 use std::path::PathBuf;
 use std::sync::mpsc::Receiver;
 
-use super::sweep::{flush_metadata, read_metadata_only, MetadataFound};
+use super::sweep::{flush_metadata, read_metadata_only, DateRead, MetadataFound};
 #[cfg(test)]
 use super::sweep::{thumbnails_outstanding, SWEEP_THUMB_SIZE};
 use super::thumbnails_fetch::{ThumbnailRequest, MAX_PREVIEW_BYTES};
@@ -226,7 +226,14 @@ pub fn spawn_thumbnails(
                     if tx.send(ThumbnailMessage::DateProgress).is_err() {
                         break;
                     }
-                    read_metadata_only(&*backend, decoder, &req, &mut found).await;
+                    if let DateRead::Offline(reason) =
+                        read_metadata_only(&*backend, decoder, &req, &mut found).await
+                    {
+                        // Said once, as the fetch loop does, so the banner goes
+                        // up and the bar stops rather than sweeping for ever.
+                        let _ = tx.send(ThumbnailMessage::Offline { reason });
+                        break;
+                    }
 
                     if found.len() >= FLUSH_EVERY {
                         flush_metadata(&catalog_path, &mut found, &tx);