heat engine : allow Properties validation failure events

Currently if properties.validate() fails causing the resource
to go to CREATE_FAILED state, we get an error when creating
the event, because the bad properties are evaluated in the event
constructor.  Instead create a dummy Error property with the
error string for the event.

ref bug 1146529

Change-Id: I12f11de617d796d65c67a5ddb6d74c86adf67b5d