engine: Improving EventQueueMonitor message

During work on the provided bug I noticed that when some event is cancelled  not well
formatted message is provided. So I fix it.

Change-Id: Idc16b4f9846b460667733e4e473bcfd89cc953b3
Bug-Url: https://bugzilla.redhat.com/895996
Signed-off-by: Michael Kublin <mkublin@redhat.com>