EAST LONDON, South Africa (Reuters) -South African mourners expressed anger and despair on Monday at the death of 21 teenagers in a tavern over the weekend, as investigating authorities said the youths were probably accidentally poisoned by something they ate, drank or smoked. The still unexplained deaths of the teens, some of whom were celebrating the end of school exams and others a birthday party, have brought an outpouring of grief and shocked a nation accustomed to injuries linked to a binge drinking culture. Authorities ruled out an earlier-touted notion of a stampede. Residents of Scene…