xpath: Improve error handling

Introduce xmlXPathSetErrorHandler allowing to set a structured error
handler for an XPath context.

Remove arguments from memory error handlers.

Use xmlRaiseMemoryError.

Remove TODO, STRANGE and CHECK_CTXT macros.

Remove remaining uses of xmlGenericError.

include/libxml/xpath.h

@@ -515,6 +515,10 @@ XMLPUBFUN xmlXPathContextPtr
 		    xmlXPathNewContext		(xmlDocPtr doc);
 XMLPUBFUN void
 		    xmlXPathFreeContext		(xmlXPathContextPtr ctxt);
+XMLPUBFUN void
+		    xmlXPathSetErrorHandler(xmlXPathContextPtr ctxt,
+					    xmlStructuredErrorFunc handler,
+					    void *context);
 XMLPUBFUN int
 		    xmlXPathContextSetCache(xmlXPathContextPtr ctxt,
 				            int active,