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.
Merge pull request #168 from alphagov/rummager_422_test

Test handling of 422 response codes from rummager