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

If a request failed with an SSL unknown certificate authority error, the
B2 backend retried indefinitely and restic would appear to hang.
It now returns the error and restic 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