DRF was swallowing an underlying error string and providing its own
which was being checked for here.
See https://github.com/encode/django-rest-framework/pull/8051 for
details.
The new version fixes that underlying issue and so the test failed.
Rather than checking for the exact string of the 404 which is not
relevant, we now just verify that the error code associated with the
request matches the HTTP response. This should make the test more
resilient to future text changes in the underlying Django function
`get_object_or_404`
bf62862b9f