Bugfix: Make `init` ignore "Access Denied" errors when creating S3 buckets

In restic 0.9.0 through 0.13.0, the `init` command ignored some permission
errors from S3 backends when trying to check for bucket existence, so that
manually created buckets with custom permissions could be used for backups.

This feature became broken in 0.14.0, but has now been restored again.

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