@Nonnull
    static <U> Promise<U> exceptionally(@Nonnull Throwable exception) {
        return HelperPromise.exceptionally(exception);
    }
Returns a Promise which is already completed with the given exception.

@param exception the exception
@param <U> the result type
@return the new completed promise
helper/src/main/java/me/lucko/helper/promise/Promise.java