Testing of limits and enabling conditions need to be done in a batched process

[chrisjrn]

At the moment everything is re-tested on adding each product. This makes testing cart category limits rather difficult, and the enabling conditions on each product gets re-tested each time. Let's make that more efficient.

[chrisjrn]

Ideas:

• set_quantities is a batch method (takes product/quantity pairs)
  • throws a single Exception type -- contains Product/error message pairs so that errors can be reported to forms
  • subclass ValidationError for ease of refactoring tests
• set_quantity is refactored to run set_quantities internally
• set_quantities runs the tests
• get_enabling_conditions returns each restriction on the products and categories provided (similar to how available_discounts works)

[chrisjrn]

Fixed.