Support broadcasts with unidentifiable areas

The original code to raise the exception was flawed: if a broadcast
only had a single area that was invalid, we would assume it was a
custom broadcast [1]. Since the recent changes [2] fixed the flaw
we're now getting exceptions for broadcasts of this kind.

It's not practical to go and manually fix the invalid broadcasts,
and the likelihood is there will be more in future as the set of
areas we support changes. This takes a pragmatic approach of simply
logging the issue and pretending such broadcasts are custom.

[1]: 926ada2f21
[2]: https://github.com/alphagov/notifications-admin/pull/4014/files#diff-2dd8f77d6df281e7674b20263cdf27a3d58b839dc5930c0087ac8b9749b313e4R92

tests/app/models/test_broadcast_message.py

@@ -1,5 +1,6 @@
 import pytest
 
+from app.broadcast_areas.models import CustomBroadcastAreas
 from app.models.broadcast_message import BroadcastMessage
 from tests import broadcast_message_json
 
@@ -70,17 +71,23 @@ def test_areas(
     assert len(list(broadcast_message.areas)) == expected_length
 
 
-def test_areas_raises_for_missing_areas():
+def test_areas_treats_missing_ids_as_custom_broadcast(notify_admin):
     broadcast_message = BroadcastMessage(broadcast_message_json(
-        area_ids=[
-            'wd20-E05009372',
-            'something else',
-        ],
+        areas={
+            'ids': [
+                'wd20-E05009372',
+                'something else',
+            ],
+            # although the IDs may no longer be usable, we can
+            # expect the broadcast to have names and polygons,
+            # which is enough to show the user something
+            'names': [
+                'wd20 name',
+                'something else name'
+            ],
+            'simple_polygons': [[[1, 2]]]
+        }
     ))
 
-    with pytest.raises(RuntimeError) as exception:
-        broadcast_message.areas
-
-    assert str(exception.value) == (
-        'BroadcastMessage has 2 areas but 1 found in the library'
-    )
+    assert len(list(broadcast_message.areas)) == 2
+    assert type(broadcast_message.areas) == CustomBroadcastAreas