bug#1305: All code that currently beeps should use visual bell instead

[Gregory Heytings]

> Eli is correct, 0.25 is far too long.  0.1 is much better.

He said 0.5 is too long, I tried 0.1, and find it too short to be noticed, 
so I tried 0.25, which seems okay from my point of view.

> In comparison with the old GNU/Linux behavior, your patch is basically 
> as good or better.  However, with your patch I have to wait until the 
> flashing is over to read the text at the bottom.  This was not the case 
> previously.  I think that would need to be fixed.

It's just a quick proof of concept, perhaps it's possible to do something 
better, I'll try to do that if there's an agreement that it's a good 
starting point.  That being said, I'd be surprised if you can move your 
eye from point to the echo area in less than 0.25 seconds.  What you 
describe is annoying only if you already look at the echo area to see the 
error.

> If we do eventually decide to go this way, perhaps we could include a 
> few variations, for example the idea of changing the mode-line text from 
> Doom could be added as optional behaviour.

FWIW, I'm not sure it's right to do that, it might interfere with packages 
which use the mode-line in exotic ways.  Moreover what's the point of 
changing the color of the mode-line?  It's distracting, what the user 
should be invited to look at at that moment is the echo area, where the 
error message is (about to be) displayed.