By default this will go to the logs. If you want to do something else with this you will need to setup error handling in your microflows. On the error handling flow you’ll have a variable called $latestError that you can query to get details of the error. This has an attribute called StackTrace that will have the message you need.

https://docs.mendix.com/refguide/error-handling-in-microflows/

I hope this helps.