The director Bartlett Sher has never, to my knowledge, pulled a sword from a stone, but for many musical theater fans his sublime revivals of classics such as “South Pacific,” “The King and I,” and “Fiddler on the Roof” have been similarly dazzling and inspiring feats. So it’s a little heartbreaking to report that his new production of “Camelot” is, not unlike King Arthur’s legendary Round Table in the show, a noble and thoughtful exercise that doesn’t succeed as brilliantly as one would have hoped.