Travel demand remains strong, but cracks are showing at certain carriers.In the recently completed first quarter, travel demand held up surprisingly well, countering investor fears that rising inflation and energy prices would eat into big-ticket purchases like airfares. But cracks are forming in individual names.Shares of Frontier Group Holdings (ULCC -3.31%) fell 25.5% in April, according to data provided by S&P Global Market Intelligence, and shares of Southwest Airlines (LUV 0.62%) and JetBlue Airways ( ...