Would be good to have more than one .bak of sessionstore through, because after crash there is nasty habit to replace bak with damaged/empty contents of main sessionstore file and bam, you lost both working version and backup.but it'd not be possible to warn beforehand about something unknown.
To wit, in Firefox something like that was implemented. Proper fix would be storing session in transactional db(like Opera Presto did and it was unbreakable), but one can dream...