core: State that vm ended with error in notifier

Currently when engine stops with error, notifier doesn't state this fact
in email subj. However it should explicitly state that VM went down with
error. This patch corrects the message.

Change-Id: Idf5668022ab00ddd4439c65c779d48843b81e3eb
Bug-Url: https://bugzilla.redhat.com/1070873
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>