Fix logging line to include response context

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.

tests/app/celery/test_research_mode_tasks.py

@@ -14,6 +14,7 @@ from app.celery.research_mode_tasks import (
     firetext_callback,
     ses_notification_callback,
     create_fake_letter_response_file,
+    HTTPError
 )
 from tests.conftest import set_config_values, Matcher
 
@@ -38,6 +39,25 @@ def test_make_mmg_callback(notify_api, rmock):
     assert json.loads(rmock.request_history[0].text)['MSISDN'] == '07700900001'
 
 
+def test_callback_logs_on_api_call_failure(notify_api, rmock, mocker):
+    endpoint = "http://localhost:6011/notifications/sms/mmg"
+    rmock.request(
+        "POST",
+        endpoint,
+        json={"error": "something went wrong"},
+        status_code=500)
+    mock_logger = mocker.patch('app.celery.tasks.current_app.logger.error')
+
+    with pytest.raises(HTTPError):
+        send_sms_response("mmg", "1234", "07700900001")
+
+    assert rmock.called
+    assert rmock.request_history[0].url == endpoint
+    mock_logger.assert_called_once_with(
+        'API POST request on http://localhost:6011/notifications/sms/mmg failed with status 500'
+    )
+
+
 @pytest.mark.parametrize("phone_number",
                          ["07700900001", "07700900002", "07700900003",
                           "07700900236"])