Suppress GWR busy-page error emails and add regression tests

update.py

@@ -14,6 +14,7 @@ from time import time
 # old authlib.jose module.  Importing authlib.deprecate first lets us insert our
 # "ignore" filter at position 0 so it wins the ordering race.
 import authlib.deprecate  # noqa: E402 — must precede the filter below
+
 warnings.filterwarnings("ignore", message="authlib.jose module is deprecated")
 
 import deepdiff
@@ -85,13 +86,20 @@ def update_gwr_advance_ticket_date(config: flask.config.Config) -> None:
     existing_html = open(filename).read()
 
     existing_dates = agenda.gwr.extract_dates(existing_html)
-    assert existing_dates
-    assert list(existing_dates.keys()) == ["Weekdays", "Saturdays", "Sundays"]
+    if existing_dates:
+        assert existing_dates
+        assert list(existing_dates.keys()) == ["Weekdays", "Saturdays", "Sundays"]
 
     new_html = requests.get(agenda.gwr.url).text
 
     new_dates = agenda.gwr.extract_dates(new_html)
     if not new_dates:
+        busy_message = (
+            "We are sorry that we cannot show you the page you were looking for. "
+            "Our website is extremely busy at the moment"
+        )
+        if busy_message in " ".join(new_html.split()):
+            return
         subject = "Error parsing GWR advance ticket booking dates"
         body = new_html
         agenda.mail.send_mail(config, subject, body)