Clean up feeds and improve test coverage

This is a fairly hefty PR to start improving our test coverage in our feeds as well as ensure errors are properly being handled.

There are quite a few fixtures which I created, hopefully what I can now do is setup fixturator to properly generate these but they will be in a separate PR.
I wonder if the test will be clearer on what's being tested if the feed's data was generated programatically. I'm not sure what approach is best, (like `$this->createBroadcast` in https://github.com/iplayer/bamboo/blob/develop/tests/Bamboo/Tests/Models/BroadcastTest.php )