Return a 404 response when a batch does not exist.

We've been getting intermittent failures in the batch import tests,
because Cucumber's PhantomJS instance was sending periodic Ajax requests
about the status of the batch it had created, even after the test
teardown had removed the batch.

Returning a 404 response instead of raising an error should stop this
causing an error in the tests, without having any effect on the
behaviour of the front-end itself (it fails just the same with a 404 or
a 500 response).
Merge pull request #437 from alphagov/import-batch-404

Return a 404 response when a batch does not exist