Print any exception that occurs in authentication callback

If an exception occurs in the python callback for openAuth()
the stack trace isn't seen by the apps, since this code is
called from libvirt context. To aid diagnostics, print the
error to stderr at least
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

python/libvirt-override.c

@@ -1776,8 +1776,10 @@ static int virConnectCredCallbackWrapper(virConnectCredentialPtr cred,
 
     PyErr_Clear();
     pyret = PyEval_CallObject(pycb, list);
-    if (PyErr_Occurred())
+    if (PyErr_Occurred()) {
+        PyErr_Print();
         goto cleanup;
+    }
 
     ret = PyLong_AsLong(pyret);
     if (ret == 0) {