[xml] enhancing the error handling API

[Stefan Seefeld <seefeld sympatico ca>]

hi there,

it has been suggested that the error reporting API
of libxml2 be enhanced, to report logical errors
instead of feeding characters into an 'error stream'.

It is said that the rework involves quite some efford,
and since Daniel mentions he has other tasks higher up
in his priority list, this needs some support from others.

I'd like to offer to help on this, and invite others to
join in, too, but want to ask Daniel for some guidance.

What are the issues involved ? What ramifications does
a change have ? What level of backward compatibility is
needed ?

And in general: what kind of error type would be most useful ?
A single 'error number' ? Or an error structure with for
example major and minor error codes, as well as other data ?

Kind regards,
                Stefan