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.
Errors returned from backend.LoadAll() were not handled, leading to
these fatal errors from the unpacker trying to read the size from the end of
an empty buffer:
`seeking to read header length failed: bytes.Reader.Seek: negative position`
This change takes care of returning on error, as well as showing which pack
failed to load and validating pack integrity.
