Kaartic: Regarding the actual patch, rather than silencing validate_worktree() (which seems an unfortunate thing to do), isn't it possible simply to do a quick test to see if the worktree directory exists before calling validate_worktree()? If it doesn't exist, then just skip down to the part of the code which does the 'prune' operation.
Nice suggestion. It seems to be a much better thing to do than silencing 'validate_worktree' (it was an "ad-hoc" patch, you see). I'll send an update incorporating this suggestion (of course, with the suggested commit message), when I find time.
Thanks, Kaartic