[xml] Catching error messages in schematron

[Stefan Behnel <stefan_ml behnel de>]

Hi,

currently (as of libxml2 2.6.29), the schematron implementation writes
validation error messages to stderr with a plain fprintf in
xmlSchematronReportOutput (happily annotated with a "TODO").

However, to make schematron usable in lxml, I need a way to propagate these
errors to the Python level. I would therefore like to see them passed into
__xmlRaiseError().

Stefan