Ensure `pool` exists before creating a flavor

Make sure the pool a flavor relies on exists before creating the flavor.
Raise 400 otherwise.

DocImpact
Flavors creation can now fail with 400 if the pool doesn't exist.

Change-Id: Ib6cc061a92b9c5bc11a3f970dd92bb8b87eb6584
Merge "Ensure `pool` exists before creating a flavor"