The music world abounds with stories of soloist mishaps, usually involving a broken string or something equally innocuous. But violinist/heartthrob Joshua Bell added an unusual chapter of his own last week in Chicago: caught up in the passion of Tchaikovsky’s Violin Concerto, Bell took too large a swipe with his bow, and with a mighty thwack, knocked over the delicate bridge that holds up his strings. (Yes, the famous Strad Bell plays on is fine.)