final CompletableFuture<AggregatedHttpResponse> toAggregatedHttpResponse(HttpResponseException cause) {
        return cause.httpResponse().aggregate(ctx.executor());
    }
Writes the {@link AggregatedHttpResponse} to the {@link Channel}.
Note that the caller has to flush the written data when needed.
core/src/main/java/com/linecorp/armeria/server/AbstractHttpResponseHandler.java