With oil hovering at $30 a barrel and gasoline below $2 a gallon, the pleasure of lower fuel prices is turning painful for more of the U.S. economy. The problem isn’t just the layoffs and investment cutbacks in the oil patch, two effects that have been expected since crude oil began sliding in 2014. Worries about energy-related bankruptcies and loan defaults also are helping to tighten financial conditions, weighing on a broader…