private HttpResponse onError(ServiceRequestContext ctx, Throwable cause) {
        if (exceptionHandler == null) {
            return HttpResponse.ofFailure(cause);
        } else {
            // TODO(ikhoon): Remove this line once the deprecated exceptionHandler has been removed.
            return exceptionHandler.handleException(ctx, ctx.request(), cause);
        }
    }
Creates a new {@link ResponseConverterFunction} instance.

@param responseConverter the function which converts an object with the configured
                         {@link ResponseConverterFunction}
rxjava2/src/main/java/com/linecorp/armeria/server/rxjava2/ObservableResponseConverterFunction.java