Raise ValueError when product has no stockrecord

When adding product to basket, Oscar expects that strategy provides
stockrecord. If there is no stockrecord, strange errors are raise, e.g.
NoneType has no attributed id.

This patch provides a more meaningful error.

Fixes #1037

oscar/apps/basket/abstract_models.py

@@ -185,6 +185,11 @@ def add_product(self, product, quantity=1, options=None):
                 "line has currency %s, while basket has currency %s") % (
                     stock_info.price.currency, price_currency))
 
+        if stock_info.stockrecord is None:
+            raise ValueError((
+                "Basket lines must all have stock records. Strategy hasn't "
+                "found any stock record for product %s") % product)
+
         # Line reference is used to distinguish between variations of the same
         # product (eg T-shirts with different personalisations)
         line_ref = self._create_line_reference(