Use Order#errors to convey useful error messages when an order fails to transition, displaying those errors appropriately in the checkout
[api] OrdersController tests must have a line item created before they can transition to address
Correct checkout_spec and state_machine_spec to account for ensure_available_shipping_rates method behaviour
[frontend] Create line items where necessary in specs
Remove deprecated Order#add_variant, Order#remove_variant and Order#shipment methods