Show separate error messages for team key

Although using a team key is functionally the same as your service being
restricted, conflating the two errors is not helpful. What we typically
saw in research was that someone was using a team key, got the error,
used a live key and got the _same_ error.

This commit adds a new error message that specifically mentions the type
of API key that you’re using.

app/notifications/rest.py

@@ -270,11 +270,15 @@ def send_notification(notification_type):
             [user.mobile_number, user.email_address] for user in service.users
         )
     ):
-        raise InvalidRequest(
-            {'to': [(
+        if (api_user.key_type == KEY_TYPE_TEAM):
+            message = 'Can’t send to this recipient using a team-only API key'
+        else:
+            message = (
                 'Can’t send to this recipient when service is in trial mode '
                 '– see https://www.notifications.service.gov.uk/trial-mode'
-            )]},
+            )
+        raise InvalidRequest(
+            {'to': [message]},
             status_code=400
         )