How to retrieve cURL response on error in PHP - php

Question

I am using PHP 7.4/8.0 and cURL to call an API and download a large PDF file based on a user request. Because the file is large, I don't want to use CURLOPT_RETURNTRANSFER and first save the file to a PHP string or a temp file, only to then send the file contents to the user's browser. Instead, I want to pass the file contents directly to the user's browser, like a proxy server might do. I am calling it with code:
<?php
$curl = curl_init();
curl_setopt_array($curl, array(
CURLOPT_URL => 'https://www.example.com/api',
CURLOPT_POST => true,
CURLOPT_POSTFIELDS => '{"item": "1"}',
CURLOPT_FAILONERROR => true,
));
curl_exec($curl);
curl_close($curl);
?>
The issue is when the API returns an error, it sends a HTTP 500 code, and in the response body, it includes the error details such as:
{
"error": "111",
"message": "This is the error message"
}
When an error happens, I want to catch that error in my code and handle it. Passing CURLOPT_FAILONERROR allows me to catch the error, but I can't figure out how to get the response body that the API sent back.
Is there a way to get the response body in a scenario like this? Again, I don't want to use CURLOPT_RETURNTRANSFER as I want to pass the output directly through to the user's browser when there is no error. I only want to get the response body when there is an error.
Thanks for your help.