Fix error message for currency mismatch

Currency of the basket and line were swapped leading to confusing error
message.

oscar/apps/basket/abstract_models.py

@@ -183,7 +183,7 @@ def add_product(self, product, quantity=1, options=None):
             raise ValueError((
                 "Basket lines must all have the same currency. Proposed "
                 "line has currency %s, while basket has currency %s") % (
-                    price_currency, stock_info.price.currency))
+                    stock_info.price.currency, price_currency))
 
         # Line reference is used to distinguish between variations of the same
         # product (eg T-shirts with different personalisations)