druntime function to query whether any exception is "in flight"

[Adam D. Ruppe]

On Tuesday, 20 October 2020 at 15:16:33 UTC, Andrei Alexandrescu 
wrote:
> One change please - can you have it return the current 
> exception as a Throwable (and null if no exception in flight)?


and prolly at least document that it shouldn't let the reference 
escape cuz otherwise it complicates memory freeing of the 
exception.

basically it would be a borrowed reference. again at least just 
documented, not necessarily enforced, but people should be aware 
escaping it is UB.