Custom form code not returning errur output (mailerlite integration)

How To
1

I’ve written this integration for Mailerlite and set it up for one of my forms:

<?php 
function my_form_custom_action( $form ) {  
  $form_fields = $form->get_fields();
  $form_id = $form_fields['formId'];

  if( $form_id !== 'yzrfqi' ) return;
  $email_address = $form->get_field_value( 'lqmhlc' );

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.mailerlite.com/api/v2/groups/XXXXXXXX/subscribers/",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => json_encode([
    'email' => $email_address,
    'enable' => true
  ]),
  CURLOPT_HTTPHEADER => [
    "X-MailerLite-ApiDocs: true",
    "X-MailerLite-ApiKey: XXXXXXXXXXXXXXXXXXXXX",
    "accept: application/json",
    "content-type: application/json"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  return $err;
} else {
  return $response;
}
  // Set result in case it fails
  $form->set_result([
    'action' => 'my_custom_action',
    'type'    => 'success', //or danger or info
    'message' => esc_html__('There was an error. Please contact us via email', 'bricks'),
  ]);

}
add_action( 'bricks/form/custom_action', 'my_form_custom_action', 10, 1 );
?>

It works like a chame. But during some testing I found out that Bricks doesn’t recognise if an error response happens. I tested with a dummy API key that would have returned an error and Bricks still thought everything went through correctly. How do I get Bricks recognise and output the error, if it happens?