bug#38457: 27.0.50; dabbrev-expand regression due to message change

[Juri Linkov]

> The importance levels for me:
>
>     (1) Inputing in minibuffer
>     (2) Inputing in buffer window
>     (3) Messages in minibuffer window
>
> I think messages(In (3)) should not disturb (1). Because they are so less 
> important. 
> And when user is busy editing in (2), he may never notice messages in
> minibuffer because they are cleared quickly.  So important messages
> should not use minibuffer.  They should use mode-line or something else.

Really important messages should be displayed permanently in a pop-up window
like display-delayed-warnings and display-warning.