rebuild=TRUE on overview lessons fails lesson check

[froggleston]

https://github.com/carpentries/sandpaper/blob/4aa05d502eeb4de64fbcd430b468e22dd4952718/R/check_lesson.R#L40C5-L40C48

Overview lessons, e.g datacarpentry/ecology-workshop, do not have episodes. If calling build_lesson(rebuild=TRUE) with overview lessons which haven't been built before, they will fail with:

! Path '/home/rstudio/lesson/episodes' does not exist
Error in report_validation(checklist, "There were errors with the lesson structure.") :
  There were errors with the lesson structure.

Solution: add overview check into the validation process.