Symfony version(s) affected: 5.2.3

Description
#32341 and #38941 started to serialize and unserialize exceptions when a message fails (using \Symfony\Component\Messenger\Transport\Serialization\Normalizer\FlattenExceptionNormalizer).

When there is another normalizer registered to handle exceptions (as in FOSRestBundle the \FOS\RestBundle\Serializer\Normalizer\FlattenExceptionNormalize), there is a conflict of normalizers.

What happens is that the FOSRestBundle serializes the exception, but later the messenger normalizer tries to deserialize it... but since it is a different format, it fails throwing an error.

(this error can heppen any time there is a custom exception normalizer registerd)

How to reproduce

• application with symfony messenger 5.x and fos rest bundle 3.x
• throw an error while consuming a message
• when the message is re-delivered an error is thrown

[2021-02-19T15:07:18.242180+00:00] console.CRITICAL: Error thrown while running command "messenger:consume". Message: "Notice: Undefined index: class" {"exception":"[object] (ErrorException(code: 0): Notice: Undefined index: class at /var/www/app/vendor/symfony/messenger/Transport/Serialization/Normalizer/FlattenExceptionNormalizer.php:74)","command":"messenger:consume","message":"Notice: Undefined index: class"} []

In FlattenExceptionNormalizer.php line 74:

  Notice: Undefined index: class  

Possible Solution

Since it is hard to have independent serializer configurations, what if the \Symfony\Component\Messenger\Transport\Serialization\Normalizer\FlattenExceptionNormalizer was just ignoring errors if the data are different from what is expected?

(from my understanding the ErrorDetailsStamp is used just to provide a nicer error message to the user).

Additional context
I've created FriendsOfSymfony/FOSRestBundle#2292 on the FOSRestBundle repo