Test for CF.MakeRequest() success

We weren't previously testing that MakeRequest() would return no errors if
the response indicated success.
Test non-JSON response bodies for CF.MakeRequest()

Put something in the body of the existing test for a response with a status
code of 500. Create an additional test for a 200 response with the same
non-JSON body, which will fail to parse.
Match specific errors from CF.MakeRequest()

To ensure that we get the error that we're expecting. This highlights one
case where we're getting the wrong kind of error - a parse error instead of
"Response body indicated failure" - which I'll fix in a subsequent commit.

These are quite verbose and could be tidied up with more "public" error
objects at a later date.
Use correct errors schema in test

Per `CloudFlareError` this field should be a slice of maps, not a slice of
strings. Otherwise we get a parse error instead of an error about the
semantic contents of the body.
Merge pull request #5 from alphagov/improve_request_tests

Improve request tests