We have been getting log lines of the following:
`API POST request on
https://api.notifications.service.gov.uk/notifications/sms/mmg failed
with None`
It's not clear what error caused the request to fail because the value
of `api_error.response` is always `None`.
There appears to be something wrong with this logging.
`raise_for_status` will raise an `HTTPError`, so then there should be no
reason to then pass that error into another `HTTPError` (which is
causing the response to be lost).
We can instead simply catch the `HTTPError` and log it's status
code.
This might not be perfect, but it's definitely an improvement and should
give us some more context about why these requests occasionally fail.