Allow GC to run if deduplication service runs into an error

What does this MR do and why?

We noticed that the Gitaly FetchIntoObjectPool RPC can fail in Gitaly Cluster. As a result, garbage collection was blocked, preventing cleanup from happening.

We now allow GC to continue if GRPC either times out or raise some internal error.

Relates to gitaly#4022 (closed)

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