Backup expiration should auto-try a deep scan if restorability requirements not met.

A rare collision of failures can cause metadata inside a backup regarding log file version continuity to be out of date. This is not easy to completely avoid, but the solution to it is to do a
`fdbbackup describe --deep`
on the backup to rescan all log files and rebuild the log continuity metadata, and it is one of the reasons this feature exists.

During backup expiration, if log continuity indicates that the restorability requirement will be broken it is generally a good idea to try a deep describe and then try the expiration again.

The expire command should just automatically do this before returning failure.