fixed mrt doesnotexists exception

@mattdrayer fixed mrt doesnotexists exception and used get_milestone instead of creating a new one every time.
Also if you look at line ~150 of manager.py, in get_course_milestones we are performing a validation on the relationship -- it must match the set of defined relationships.  I'm not entirely sure if this is the right way to go, because it means a code change every time a new relationship type is added to the system.  Maybe the right thing to do inside of the validator is to pull the (distinct) list of MRTs from data.py and check the incoming relationship against that list...which is a lot of work for what could be handled w/ the invalid MRT exception approach.  Let me know what you think.