The relentless churn of famous figures who’ve hosted Saturday Night Live over the years has left the show with not a few regrets. Being a bad host is one thing, with SNL’s shark-like momentum simply shaking off a bummer of an episode and moving on. (Or, as in the cases of such universally reviled outings from hosts Milton Berle and Frank Zappa, being pulled from syndication by producer Lorne Michaels for a time.) But then there are the host bookings that, thanks to the host’s life away from the SNL spotlight, leave a stain that Michaels and NBC would much...