[Xcb] XCB error handling; my penultimate proposal

[Vincent Torri]

>
> Correct me if I'm wrong, Bart: you feel at this point it's too late to
> make backwards-incompatible changes that affect most of the code that
> uses XCB. I disagree, and I really want a clean API -- enough to
> inconvenience the good people who have been kind enough to test out and
> improve this code for us. I think they will forgive me, but I'd be happy
> to make the necessary changes to their source by hand myself if that's
> what it takes to release with a satisfying API.

i agree. I think that xcb is not widely used. If changes has to be done,
it's now.

Vincent