Deliver a warning if attempting to create an invalid taxed_product from factory

spec/factories.rb

@@ -200,6 +200,7 @@ FactoryGirl.define do
     tax_category { create(:tax_category) }
 
     after(:create) do |product, proxy|
+      raise "taxed_product factory requires a zone" unless proxy.zone
       create(:tax_rate, amount: proxy.tax_rate_amount, tax_category: product.tax_category, included_in_price: true, calculator: Spree::Calculator::DefaultTax.new, zone: proxy.zone)
     end
   end