Move translation from expat.error to SAXParseException into feed, so that

Move translation from expat.error to SAXParseException into feed, so that
callers of feed will get a SAXException.
In close, feed the last chunk first before calling endDocument, so that
the parser may report errors before the end of the document. Don't do
anything in a nested parser.
Don't call endDocument in parse; that will be called in close.
Use self._source for finding the SystemID; XML_GetBase will be cleared in
case of an error.