handle doc dl connection errors correctly

previously we'd see an error message in the logs:
`AttributeError: 'NoneType' object has no attribute 'status_code'`
because we were assuming the requests exception would always have a
response - it won't have a response if it wasn't able to create a
connection at all.

app/clients/document_download.py

@@ -42,7 +42,9 @@ class DocumentDownloadClient:
             # if doc dl responds with a non-400, (eg 403) it's referring to credentials that the API and Doc DL use.
             # we don't want to tell users about that, so anything that isn't a 400 (virus scan failed or file type
             # unrecognised) should be raised as a 500 internal server error here.
-            if e.response.status_code == 400:
+            if e.response is None:
+                raise Exception(f'Unhandled document download error: {repr(e)}')
+            elif e.response.status_code == 400:
                 error = DocumentDownloadError.from_exception(e)
                 current_app.logger.info(
                     'Document download request failed with error: {}'.format(error.message)

tests/app/clients/test_document_download.py

@@ -1,3 +1,4 @@
+import requests
 import requests_mock
 import pytest
 
@@ -56,3 +57,16 @@ def test_should_raise_non_400_statuses_as_exceptions(document_download):
 
     assert type(excinfo.value) == Exception  # make sure it's a base exception, so will be handled as a 500 by v2 api
     assert str(excinfo.value) == 'Unhandled document download error: {"error": "Auth Error Of Some Kind"}'
+
+
+def test_should_raise_exceptions_without_http_response_bodies_as_exceptions(document_download):
+    with pytest.raises(Exception) as excinfo, requests_mock.Mocker() as request_mock:
+        request_mock.post(
+            'https://document-download/services/service-id/documents',
+            exc=requests.exceptions.ConnectTimeout
+        )
+
+        document_download.upload_document('service-id', 'abababab')
+
+    assert type(excinfo.value) == Exception  # make sure it's a base exception, so will be handled as a 500 by v2 api
+    assert str(excinfo.value) == 'Unhandled document download error: ConnectTimeout()'