President Donald Trump’s decision Wednesday to impose a 30-day ban on most Europeans entering the United States is the latest stunning setback for an airline industry that is reeling from a drop in bookings and a surge in people who are canceling reservations for fear of contracting the coronavirus. In his address from the Oval Office, Trump said U.S. restrictions on people coming from China and other countries with early outbreaks of COVID-19 had held down the number of cases in the United States compared with Europe.