Gracefully exit if we can't find a date

If the release name passed into the tool doesn't yet have a schedule,
which is actually the most common use case, we won't be able to
auto-detect a date.  In that case let's bail somewhat nicely.

Change-Id: I4072e73c15d0299755fcce43f8b35798179719c3

openstack_election/cmds/setup_election_config.py

@@ -140,6 +140,9 @@ def main():
     if (args.date is None):
         schedule = utils.get_schedule_data(names[idx+1])
         args.date = select_release_end_date(schedule)
+        if args.date is None:
+            print("Error: no end date found in series data")
+            exit(1)
 
     # Given the release history:
     #  Stein, Rocky[0], Queens[1], Pike[2], Ocata[3]