Log the response we get back from Backdrop

It makes it easier to diagnose why calls to Backdrop fail if we include
the JSON response that Backdrop returned with.

I've also taken the ``requests.post`` line out of the exception handler
because any exception coming from here would be unexpecetd - ie socket
failure, network problem, rather than a well-understood Backdrop-originating
error.
Merge pull request #44 from alphagov/improve-logging

Log the response we get back from Backdrop