If someone tries to cancel a broadcast but the references don’t match
and existing broadcast we correctly return a 404.
If they don’t provide any references then we get an exception. This
commit catches the missing references and returns a 400. I think this
is more appropriate because it’s malformed request, rather than a
well-formed request that doesn’t match our data. It also lets us write a
more specific and helpful error message.
bbc444699a