clawhdf5-remote: a failed first fetch is Error::Remote

cached() mapped an error of its open-time prefetch (a network error, a
changed file) to Error::Hdf5(Format(Storage)), misclassifying it for
callers that match on the variant. It is now Error::Remote
(RemoteError::Backend with the backend's message). open_object and the
s3/gs/az URLs fetch the first block of an ObjectStoreStorage directly, so
their errors keep their kind (FileChanged, ObjectStore).

Test: cached() over a backend whose reads fail gives Error::Remote(Backend)
(Error::Hdf5 before).

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-remote/tests/http.rs

@@ -834,3 +834,27 @@ fn slow_links_read_and_stalled_ones_fail() {
     assert!(matches!(e, Error::Remote(RemoteError::Transport(_))), "{e}");
     assert!(t.elapsed() < Duration::from_secs(5), "{:?}", t.elapsed());
 }
+
+/// `cached` reports a failed first fetch as a remote error, not as an
+/// HDF5 format error.
+#[test]
+fn cached_reports_a_failed_prefetch_as_remote() {
+    use clawhdf5_format::error::FormatError;
+    use std::borrow::Cow;
+    struct Down;
+    impl clawhdf5_format::storage::Storage for Down {
+        fn read_at(&self, _: u64, _: usize) -> Result<Cow<'_, [u8]>, FormatError> {
+            Err(RemoteError::Transport("connection refused".into()).into())
+        }
+        fn len(&self) -> u64 {
+            1 << 20
+        }
+    }
+    let e = clawhdf5_remote::cached(Box::new(Down), &Options::default())
+        .map(|_| ())
+        .unwrap_err();
+    assert!(
+        matches!(&e, Error::Remote(RemoteError::Backend(m)) if m.contains("connection refused")),
+        "{e:?}"
+    );
+}