This is a new error which implements the restic.Fataler interface.
Errors of this type are written to stderr, the restic exits. For all
other errors, restic prints the stack trace (if available).
This code reads all pack headers from all packs and rebuilds the index
from scratch. Afterwards, all indexes are removed. This is needed
because in #434 the command `optimize` produced a broken index that
did not contain a blob any more. Running `rebuild-index` should fix
this.
I had the suspicion that one of my repositories contained redundant
data blobs that is not recorder in the index. In order to check this I
needed to dump information about the pack files without consulting the
index. The code here iterates over all packs and dumps the headers, and
dumps information about them in JSON (so we can use other tools to parse
that information again).
