archiver: fix reporting for check_current_backup

archiver: fix reporting for check_current_backup

It's getting a bit more complex here.

Basic idea behind is - check_current_backup() should not
log error when a user is using a read-only filesystem,
so e.g.  vgscan  will not report any error when it tries
to take missing backup.

We still have cases when error could be reported though,
e.g. the backup this would be a symbolic link, but these
are rather misconfiguration and unexpected case.