a gs service account may only have object permissions on an existing
bucket but no bucket create/get permissions.
these service accounts currently are blocked from initialization a
restic repository because restic can not determine if the bucket exists.
this PR updates the logic to assume the bucket exists when the bucket
attribute request results in a permissions denied error.
this way, restic can still initialize a repository if the service
account does have object permissions
fixes: https://github.com/restic/restic/issues/3100
