Bugfix: Fix hang with Backblaze B2 on SSL certificate authority error

Previously, if a request failed with an SSL unknown certificate authority
error, the B2 backend retried indefinitely and restic would appear to hang.

This has now been fixed and restic instead fails with an error message.

https://github.com/restic/restic/issues/3556
https://github.com/restic/restic/issues/2355
https://github.com/restic/restic/pull/3571
