For the most part, “Age of Terror: Art since 9/11” is sobering, serious stuff. Still, the British press does not seem have been especially impressed. More than one review has criticized the show and even its premise as trite or banal. Some have argued that art is simply unequal to the magnitude of the event. But isn’t that a given? Who can forget Karlheinz Stockhausen’s shocking observation, six days after the Twin Towers fell, that the attack was “the greatest work of art imaginable for the whole cosmos”?