The old behavior was problematic in the context of rebuild-index as it
could leave old, possibly invalid index files behind without returning a
fatal error.
Prune calls rebuildIndex before removing any data from the repository.
For this use case failing to delete an old index MUST be treated as a
fatal error. Otherwise the index could still contain an old index file
that refers to blobs/packs that were later on deleted by prune. Later
backup runs will assume that the affected blobs already exist in the
repository which results in a backup which misses data.
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