Handle pack loading errors in rebuild-index
Handle pack loading errors in rebuild-index

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.

byo/restic,middelink/restic,fawick/restic,mappu/restic,ckemper67/restic,fawick/restic,stuertz/restic,kurin/restic,ar-jan/restic,middelink/restic,ar-jan/restic,restic/restic,kurin/restic,arithmetric/restic,bchapuis/restic,byo/restic,jayme-github/restic,restic/restic,stuertz/restic,jayme-github/restic