Provide an error message when user specifies a non-supported distribution for a loop iteration count

Currently, loop iteration count only supports a IntPMF function as random variable. If anything other is specified by the user, DML2QPN will generate an invalid model and SimQPN will fail with the following error message:

ERROR Missing color references!
Details:
  place-num = 9
  place.id = _42
  place.name = place_loop_pool_BasicComponent_name:/wildfly-ejb-multi-server-app-web_id:_NlWZcbFaEeScNP3zuqaoeQ//FineGrainedBehavior//ComponentInternalBehavior//LoopAction#1
  place.departure-discipline = NORMAL