Differentiate synchronous and asynchronous error of an AIO operation

[Alan Bateman]

Zhong Yu wrote:
> I suspect that an AIO operation is usually the
> last call
>
>     lock
>     ...
>     operation( handler );
>     // nothing in between
>     unlock
>
>   
Yes, I expect we will see this pattern quite often and probably with 
handler replaced by "this", meaning the same completion handler instance 
will be invoked to deal with the result.

-Alan