In his finely wrought portrait, “Maestro: Leonard Bernstein,’’ Hershey Felder taps into the ineradicable sadness his Lawrence-born, Harvard-educated subject grappled with as he considered his own unfulfilled expectations. It wasn’t that Bernstein developed a Marlon Brando-like contempt for his craft, but rather that, like Norman Mailer — another man of multifarious talents and appetites — he allowed himself to be pulled in too many directions.