public static <U> UnmodifiableFuture<U> exceptionallyCompletedFuture(Throwable cause) {
        requireNonNull(cause, "cause");
        final UnmodifiableFuture<U> future = new UnmodifiableFuture<>();
        future.doCompleteExceptionally(cause);
        return future;
    }
Returns an {@link UnmodifiableFuture} which has been completed exceptionally with the specified
{@link Throwable}.
core/src/main/java/com/linecorp/armeria/common/util/UnmodifiableFuture.java