backup log files on resubmission

Another convenience feature request: when resubmitting a failed (or terminated due to scheduling limits) run, currently I have to manually remember to first backup the previous log file if I ever want to check it again. It would be nice if at startup the run would notice that it's intended logfile path already has a file in it, then back that up. To be safe against multiple resubmissions, it could just add .0, .1, etc incrementally by checking in a quick loop for the lowest of those that does not yet exist.