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>