Returns a new CompletionStage that is already completed
exceptionally with the given exception and supports only those
methods in interface {@link CompletionStage}.
@param ex the exception
@param <U> the type of the value
@return the exceptionally completed CompletionStage
Returns a new CompletionStage that is already completed exceptionally with the given exception and supports only those methods in interface {@link CompletionStage}.
@param ex the exception @param <U> the type of the value @return the exceptionally completed CompletionStage