Bugfix: Restic init ignores "Access Denied" errors when creating an S3 bucket

In restic 0.9.0 through 0.13.0, some permission errors from S3 backends where ignored
when trying to check the bucket existence, so that manually created buckets with custom permissions
could be used for backups. This feature was broken in 0.14.0, but is now restored.

https://github.com/restic/restic/issues/4085
https://github.com/restic/restic/pull/4086