RE: [xml] Error handling with the XmlReader API

["Stéphane Bidoul" <stephane bidoul softwareag com>]

>   Okay, looks good, compiles and regression tests are fine. I added
> readererr.py to the python tests. My only little concern is about
> varargs, I hope you followed the existing code in error.c because I
> remember it took a few iterations before getting reliable and portable
> code for varargs error handlers...

I copied buildMessage from libxml.c in the python bindings,
but it looks very similar to the XML_GET_VAR_STR macro in errors.c. 
<stdarg.h> is required for compile, as in error.c... 

While coding this, I noticed I was providing a distinction
between validation errors and other errors. I did not provide
such a distinction for the xmlParserCtxt error handlers
at the python level. 
Is such a distinction useful?
Should I add the 2 necessary callbacks for the xmlParserCtxt 
python bindings?

-sbi