Get last tried backup date in the UI

Hello,

I'd like to suggest a feature request for Burp-UI v0.7.0, altough I'm pretty sure I already have read something like this here before. Currently, th e only date we have is the date from the last backup. It would be nice to also have the date from the last time the client tried to backup (eg the working/timestamp file).

This would help identifying machines that have large backups vs machines that just don't communicate.