[8.2.0] Don't fetch empty digest on failure status in `FetchBlobResponse` (#25277)

When the `GrpcRemoteDownloader` receives a `FetchBlobResponse` with an
error status (instead of a connection level status error), it must fail
and fall back to the HTTP downloader if configured instead of ignoring
the status and attempting to fetch the (usually empty) digest from the
cache. The previous behavior could result in errors being masked and
resulting in an empty file being downloaded if the remote cache accepts
empty `Digest` messages.

Closes #25244.

PiperOrigin-RevId: 725963696
Change-Id: I9f643b944b9fff9264c72f41485564ff1fab71d9

Commit
https://github.com/bazelbuild/bazel/commit/e0cd2f245491f93065e5e04106a9b6e7eb029292



Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>
Co-authored-by: Ian (Hee) Cha <heec@google.com>