bug#40774: Error messages shouldn't be hidden when the user is idle

[ndame]

>  The method Emacs uses to
> determine when to run the next idle timer will not work,

It just occurred to me there is no need for idle check at all.

The echo area is currently automatically cleared when the user
performs a command. Otherwise, a new message can override
the echo area.

Only the latter needs to be changed, so that a new message does
not erase error messages.

So if emacs is left alone then arriving error messages are
accumulated and shown in a multiline echo area. If a new regular
message arrives then it is shown at the bottom of the multiline
echo area with the earlier error messages above.

And when the user comes back and performs a command then
the echo area is cleared as usual.

So the only change needed is that error messages can only be cleared
from the echo area by the user doing some command. Otherwise they
are collected and shown.