* python/libxml.c: register xmlSchemaSetValidErrors, patch from
  Brent Hendricks in the mailing-list
* include/libxml/valid.h HTMLparser.c SAX2.c valid.c
  parserInternals.c: fix #156626 and more generally how to find out
  if a validation contect is part of a parsing context or not. This
  can probably be improved to make 100% sure that vctxt->userData
  is the parser context too. It's a bit hairy because we can't
  change the xmlValidCtxt structure without breaking the ABI since
  this change xmlParserCtxt information indexes.
Daniel