[Xcb] event/error tags

[Jeremy A. Kolb]

I've been thinking that if we're going to eventually have documentation in 
the XML that we might want to add some tags to specify that certain 
requests might generate errors or events.  Not only would this help with 
documentation but it might make it clearer to the people writing: a way to 
keep track of what events or errors are needed for a particular extension.  
Just something to toss around.  What do you all think?

Jeremy