The buddy system. Last night’s Prom was as close to an all-smiles evening as one could hope for with rain pouring down all day. David Robertson, although known as a champion of contemporary music, programmed two easy pieces, the Barber Violin Concerto, which is about as challenging as a box of caramels (very delicious caramels) and the Sibelius Second Symphony, a sure-fire hit in Nordic-friendly Britain. There are so many stories of promising American conductors who falter in middle age (Robertson turned 52 last month) that I was eager to hear him a second time. The first was with the Boston Symphony some years ago. Before I register my impressions, however, there’s a spic-and-span back story to his career — apparently this man has left behind him a trail of good will wherever he goes. He looks fit and friendly, with flat gray hair and the long face of a Yankee banker sitting for a Copley portrait. Born and raised in Malibu — not an arduous beginning, one assumes — Robertson was educated at the Royal Academy of Music. This tie to London glided into becoming the chief guest conductor of the BBC Symphony, which he presided over last night with happy faces all around. Robertson even entered the thorny patch that is the Ensemble Intercomtemporain in Paris and was cheered on despite having no ties to its founder, the formidable Pierre Boulez. Robertson preferred to conduct John Adams instead, and he got away with it.