The Boston Celtics bowed out of the postseason Saturday night after losing Game 7 of their opening-round series at home against the 76ers. A season that was so promising collapsed as Boston blew a 3-1 lead. With Jayson Tatum out for most of the regular season while recovering from a torn Achilles tendon, the Celtics deftly managed his absence and remained one of the NBA’s best teams. They earned the No. 2 seed in the Eastern Conference and entered the playoffs buoyed by Tatum’s return. They looked like a contender, but fell apart when it mattered most. It was a tale of two seasons for Boston. The regular season was a clear success, while the postseason was a mess. Let's take a look at what happened and hand out some grades. Regular season: A- With Tatum out, few expected the Celtics to compete for the best record in the Eastern Conference, but that’s exactly what they did. Boston finished the regular season 56-26, four games behind the first-place Pistons, and in possession of the No. 2 seed in the East.…