Show users more detailed validation failed mesage on notification page

Show valdiation failed messages on letter notifications in red text,
not in the banner like we do on Uploads and Validation checker pages.
This is because it is a different step in the journey: the user
has already sent the notification and styling needs to be in line
with other places where user is checking the notification she already
has sent.

app/notify_client/notification_api_client.py

@@ -103,17 +103,6 @@ class NotificationApiClient(NotifyAdminAPIClient):
 
         return self.get(url=get_url)
 
-    def get_notification_letter_preview_with_overlay(self, service_id, notification_id, file_type, page=None):
-        get_url = '/service/{}/template/preview/{}/{}{}{}'.format(
-            service_id,
-            notification_id,
-            file_type,
-            '?overlay=1',
-            '&page={}'.format(page) if page else '',
-        )
-
-        return self.get(url=get_url)
-
     def update_notification_to_cancelled(self, service_id, notification_id):
         return self.post(
             url='/service/{}/notifications/{}/cancel'.format(service_id, notification_id),