bug symfony#52095 [Notifier][Sendinblue] Handle error responses without a message key (stof)

This PR was merged into the 5.4 branch.

Discussion
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[Notifier][Sendinblue] Handle error responses without a message key

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT

During their outage yesterday where the endpoint returned errors with status code 500, there was no `message` key in the json response.
This avoids a notice when the key does not exist. Instead, it uses the full response content, as done when the decoding fails.

Commits
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8ae8993 Handle Sendinblue error responses without a message key

Author: nicolas-grekas

src/Symfony/Component/Mailer/Bridge/Sendinblue/Transport/SendinblueApiTransport.php

@@ -65,7 +65,7 @@ protected function doSendApi(SentMessage $sentMessage, Email $email, Envelope $e
         }
 
         if (201 !== $statusCode) {
-            throw new HttpTransportException('Unable to send an email: '.$result['message'].sprintf(' (code %d).', $statusCode), $response);
+            throw new HttpTransportException('Unable to send an email: '.($result['message'] ?? $response->getContent(false)).sprintf(' (code %d).', $statusCode), $response);
         }
 
         $sentMessage->setMessageId($result['messageId']);

src/Symfony/Component/Notifier/Bridge/Sendinblue/SendinblueTransport.php

@@ -75,7 +75,7 @@ protected function doSend(MessageInterface $message): SentMessage
         if (201 !== $statusCode) {
             $error = $response->toArray(false);
 
-            throw new TransportException('Unable to send the SMS: '.$error['message'], $response);
+            throw new TransportException('Unable to send the SMS: '.($error['message'] ?? $response->getContent(false)), $response);
         }
 
         $success = $response->toArray(false);