@Override
        public boolean completeExceptionally(Throwable ex) {
            if (!super.completeExceptionally(ex)) {
                return false;
            }

            handlePendingPiggybacks(null);
            return true;
        }
A {@code Consumer<PooledChannel>} if only 1 handler.
A {@code List<Consumer<PooledChannel>>} if there are 2+ handlers.
core/src/main/java/com/linecorp/armeria/client/HttpChannelPool.java