fixed mrt doesnotexists exception

@mattdrayer fixed mrt doesnotexists exception and used get_milestone instead of creating a new one every time.
well I'm handling internal.MilestoneRelationshipType.DoesNotExist exception for those scenarios when we try to get pre-requisite course for course at a point when there is no data in the tables. Like for the first time course author try to set a pre-requisite course. In this case caller should be return `None` because there is nothing in the database.