Better exception message header for DLT

[ClearClean]

Expected Behavior

In DeadLetter messages, the header KafkaHeaders.DLT_EXCEPTION_MESSAGE should have message from at least the root cause exception.

Current Behavior

The header KafkaHeaders.DLT_EXCEPTION_MESSAGE is always "Listener failed" which is not really useful.

Context

Since spring framework 6.0, we do not have the message from the cause chain anymore.

We can not get any useful information from this header anymore.

[garyrussell]

Thanks for reporting; in the meantime, you should be able to find the root cause in the stack trace header.