Test handling of 422 response codes from rummager
Test handling of 422 response codes from rummager

We're going to change rummager to return 422 for validation errors,
following https://github.com/alphagov/styleguides/blob/master/api.md

This adds a test that we get an appropriate exception as a result.

alphagov/gds-api-adapters,bitzesty/gds-api-adapters,bitzesty/gds-api-adapters