Stack trace in `Exception.msg` gives  poor UX
Async stack traces should not reside in the `msg` field of the exception - this field is meant for a short, human-readable description of the error that can be logged on a single line.

The Exception object already contains a `parent` field for exceptions - this, together with a dedicated FutureError exception that has a `source` field for the purpose would allow code that logs the exception to deliver a better UX.
