capture errors from libxml2

fixes #4

error_thunk.go

@@ -1,13 +1,31 @@
 package xmlsec
 
-// onError_cgo is a C function that can be passed to xmlSecErrorsSetCallback which
-// in turn invokes the go function onError which captures errors generated by
-// libxmlsec.
+// #include <stdio.h>
+// #include <stdarg.h>
+// #include <libxml/parser.h>
+// #include <libxml/parserInternals.h>
+// #include <libxml/xmlmemory.h>
+// #include <xmlsec/xmlsec.h>
+// #include <xmlsec/errors.h>
 //
-// For reasons I do not completely understand, it must be defined in a different
-// file from onError.
-
-// void onError_cgo(char *file, int line, char *funcName, char *errorObject, char *errorSubject, int reason, char *msg) {
-//   onError(file, line, funcName, errorObject, errorSubject, reason, msg);
+// void onXmlError(const char *msg);  // implemented in go
+// void onXmlsecError(const char *file, int line, const char *funcName, const char *errorObject, const char *errorSubject, int reason, const char *msg);  // implemented in go
+//
+// static void onXmlGenericError_cgo(void *ctx, const char *format, ...) {
+// 	char buffer[256];
+// 	va_list args;
+// 	va_start(args, format);
+// 	vsnprintf(buffer, 256, format, args);
+// 	va_end (args);
+//  onXmlError(buffer);
+// }
+//
+// static void onXmlsecError_cgo(const char *file, int line, const char *funcName, const char *errorObject, const char *errorSubject, int reason, const char *msg) {
+// 	onXmlsecError(file, line, funcName, errorObject, errorSubject, reason, msg);
+// }
+//
+// void captureXmlErrors() {
+// 	xmlSecErrorsSetCallback(onXmlsecError_cgo);
+// 	xmlSetGenericErrorFunc(NULL, onXmlGenericError_cgo);
 // }
 import "C"