[Xcb] Naming standard for X error constants

[Ian Osgood]

Jamey reverted the experimental patch to qualify error constants with  
XCB_BAD_*, because some of the extensions were already doing this  
work by hand, resulting in XCB_BAD_BAD_* for their errors. (I don't  
know why Jamey didn't just fix the extensions.) I would like to  
figure out how to qualify error constants once and for all before we  
make another release.

My next proposal is to qualify errors by appending *_ERROR. Can  
anyone think of a reason this would not work or otherwise be hard to  
implement?

Really, I don't see how we could recommend XCB for production code  
(which unlike demo code, does strict error checking) until we  
finalize this. In my opinion, it is not feasible to leave the error  
constants unqualified as they are.

Ian