A crisis at South Africa’s dominant electricity supplier, Eskom Holdings SOC Ltd., threatens the nation’s economy. The state-owned utility can’t cover its costs and make the huge interest payments on its mountain of debt. Its poorly maintained power plants are struggling to meet demand, and it has gone through 10 chief executive officers in a decade. Goldman Sachs Group Inc. describes Eskom as the country’s biggest financial risk. The government has increased and accelerated a bailout for the ut