Raise error if multiple broadcasts found for reference

Because the `<reference>` field of a `cancel` message can contain an
arbitrary number of items it’s possible for it to reference more than
one current alert.

In this case it is ambiguous which alert should be cancelled, so we
should raise a custom error.

This will help people know that they have to manually go into Notify and
figure out which alert(s) to cancel there.

tests/app/v2/broadcast/sample_cap_xml_documents.py

@@ -79,6 +79,15 @@ WAINFLEET_CANCEL_WITH_EMPTY_REFERENCES = WAINFLEET_CANCEL.format(
     "<references></references>"
 )
 
+WAINFLEET_CANCEL_WITH_WINDMERE_REFERENCES = WAINFLEET_CANCEL.format(
+    "<references>"
+    # Wainfleet
+    "www.gov.uk/environment-agency,50385fcb0ab7aa447bbd46d848ce8466E,2020-02-16T23:01:13-00:00,"
+    # Windemere
+    "www.gov.uk/environment-agency,4f6d28b10ab7aa447bbd46d85f1e9effE,2020-02-16T19:20:03+00:00"
+    "</references>"
+)
+
 UPDATE = """
     <alert xmlns="urn:oasis:names:tc:emergency:cap:1.2">
       <identifier>PAAQ-4-mg5a94</identifier>

tests/app/v2/broadcast/test_post_broadcast.py

@@ -220,6 +220,38 @@ def test_cancel_request_does_not_cancel_broadcast_if_reference_does_not_match(
     assert response_for_cancel_json["errors"] == expected_error
 
 
+def test_cancel_raises_error_if_multiple_broadcasts_referenced(
+    client,
+    sample_broadcast_service,
+):
+    auth_header = create_service_authorization_header(service_id=sample_broadcast_service.id)
+
+    for cap_document in (
+        sample_cap_xml_documents.WAINFLEET,
+        sample_cap_xml_documents.WINDEMERE,
+    ):
+        response_for_create = client.post(
+            path='/v2/broadcast',
+            data=cap_document,
+            headers=[('Content-Type', 'application/cap+xml'), auth_header],
+        )
+        assert response_for_create.status_code == 201
+
+    # try to cancel two broadcasts with one request
+    response_for_cancel = client.post(
+        path='/v2/broadcast',
+        data=sample_cap_xml_documents.WAINFLEET_CANCEL_WITH_WINDMERE_REFERENCES,
+        headers=[('Content-Type', 'application/cap+xml'), auth_header],
+    )
+    response_for_cancel_json = json.loads(response_for_cancel.get_data(as_text=True))
+
+    assert response_for_cancel.status_code == 400
+    assert response_for_cancel_json["errors"] == [{
+        'error': 'BadRequestError',
+        'message': 'Multiple alerts found - unclear which one to cancel',
+    }]
+
+
 def test_cancel_request_does_not_cancel_broadcast_if_service_id_does_not_match(
     client,
     sample_broadcast_service,