clawhdf5-remote: readers waiting on a failed fetch get its error

A reader that waited for another reader's fetch of a block got "the
fetch of this block failed" when that fetch failed, not why: a file
replaced on the server while open was reported as FileChanged to one
thread and as an anonymous failure to the others. The fetch's error is
now handed to every reader waiting on it.

Regression test: four threads read the same block from a slow backend
whose fetches fail with a "changed while open" error; each gets that
error (it failed for the waiters before this change).

Also fixes the ignore-Range test, broken by the previous commit: the
test server now counts a body before sending it, so "the refused body
was not read" is checked as "refused at the first response".

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

crates/clawhdf5-remote/tests/http.rs

@@ -301,11 +301,7 @@ fn a_server_that_ignores_range_is_refused_or_downloaded_when_allowed() {
         matches!(err, Error::Remote(RemoteError::RangeNotSupported(_))),
         "{err}"
     );
-    assert!(
-        server.bytes() < bytes.len() as u64,
-        "refusing must not download the file ({} bytes read)",
-        server.bytes()
-    );
+    assert_eq!(server.requests(), 1, "refused at the first response");
     let mut opts = quick();
     opts.http.allow_full_download = true;
     let storage = storage_for_url(&url, &opts).unwrap();