Consider renaming the ConstraintsException

[pjhartzell]

Product backend implementations are instructed to raise the the ConstraintsException when an OpportunityRequest or an OrderPayload does not validate. Does this mean that only the CQL2 filter constraints within these objects are to be validated? If so, we should make that clear in both docstrings. If not, from a terminology standpoint, raising a ConstraintsException when an opportunity request or an order payload does not validate is confusing, and renaming the exception to something more generic like InvalidRequestError could improve clarity.