The lead singer for one of the best bands of the last quarter-century had a powerful message for fans. On Aug. 23, in a packed venue filled with approximately 28,000 concertgoers in Hershey, Pennsylvania, front man Brad Arnold of 3 Doors Down took a few minutes between songs to talk about the most important thing any of them will ever hear. “Something was put on my heart two years ago,” Arnold said in a video posted to YouTube. Before playing the song “Away from the Sun,” which he called his personal favorite, the singer shared a message. He explained that he identified with the despair-filled…