If A Late Quartet, the second feature by Yaron Zilberman and his first film since his 2004 documentary Watermarks, proves anything, it’s that good performances can elevate average movies. To say that Zilberman’s cast saves his picture would be somewhat generous, though. They only distract us from its inadequacies, which are numerous, though perhaps this is a harsher judgment than the film really deserves. A Late Quartet is harmless, airy fluff, small-scale prestige cinema that smartly gathers together a group of very gifted actors in the service of exploring life lessons filtered through the overarching motif of Beethoven’s Opus 131 String Quartet (in C-sharp minor); it’s also painfully undercooked to…