“ER” couldn’t say goodbye without visits from George Clooney and Julianna Marguilies, and writer-director John Wells crafted a beautiful episode that brought Doug Ross and Carol Hathaway, the show’s beloved couple who still have that wonderful chemistry, back into the storyline. The emotional episode about transplants, so true to the essence of TV’s best hospital drama, also reunited Eric La Salle’s Benton with Noah Wylie’s Carter, and included sparkling guest performances from Susan Sarandon as the grandmother of a dying child and Christian Clemsenson (Jerry from “Boston Legal”) as a cranky surgeon. “ER,” which wraps up on April 2, is giving fans a cherished swan song.