Add an error message when user creates aggregation stat having only 1 num stat or even without any of them

[worstcop]

When user creates a new game he could create tree types of stats: numerable, enumerable and aggregate (this one on induvidial page). If user create only enumerable stat or only 1 type of numerable stat he couldn't create correctly aggregate stat. So on aggregate stat page when he click on add button, having inputing data before, he should get an error message to add at least 2 numerable stats to create an aggregate stat.