public AsyncLoaderBuilder<T> exceptionHandler(BiFunction<? super Throwable, ? super @Nullable T,
            ? extends @Nullable CompletableFuture<T>> exceptionHandler) {
        requireNonNull(exceptionHandler, "exceptionHandler");
        this.exceptionHandler = exceptionHandler;
        return this;
    }
Handles the exception thrown by the loader function.
If the exception handler returns {@code null}, {@link AsyncLoader#load()} completes exceptionally.
core/src/main/java/com/linecorp/armeria/common/util/AsyncLoaderBuilder.java