Robert Morse died at 90 recently, on April 20. Decades before introducing himself to millions as the eccentric, bow-tied, frequently barefoot advertising honcho in “Mad Men,” he built a zigzagging, often frustrated career on a particular mad charm entirely his own. An imp’s imp, Morse was one of the screen’s handful of hallowed, indelible gap-toothed comic personalities (the British actor Terry-Thomas was another), though he was most at home on stage. Six years and an entire cultural epoch separated Morse’s Tony Award-winning turn as J. Pierrepont Finch, the beguilingly inhuman success story d…