[round_robin_calendar] The script fails to update the triage owners when there is an invalid fallback for one of the calendars

2025-01-13 12:00:37,778 - ERROR - Rule triage_owner_rotations
Traceback (most recent call last):
  File "/home/rm_bot/bugbot/bugbot/bzcleaner.py", line 856, in run
    self.send_email(date=date)
  File "/home/rm_bot/bugbot/bugbot/bzcleaner.py", line 772, in send_email
    data = self.get_email_data(date)
  File "/home/rm_bot/bugbot/bugbot/rules/triage_owner_rotations.py", line 87, in get_email_data
    new_triagers = self.component_triagers.get_new_triage_owners()
  File "/home/rm_bot/bugbot/bugbot/component_triagers.py", line 87, in get_new_triage_owners
    new_triager = self.round_robin.get(
  File "/home/rm_bot/bugbot/bugbot/round_robin.py", line 162, in get
    fb = cal.get_fallback_bzmail()
  File "/home/rm_bot/bugbot/bugbot/round_robin_calendar.py", line 49, in get_fallback_bzmail
    raise BadFallback("'{}' is an invalid fallback".format(self.fallback))
bugbot.round_robin_calendar.BadFallback: 'XXXX XXXX' is an invalid fallback

The failure should be confined to the relevant calendar and only occur when fallback is needed.

Start with bugbot/round_robin_calendar.py and follow the call path through bugbot/round_robin.py and bugbot/component_triagers.py shown in the traceback. Reproduce an invalid fallback during fallback selection and verify that only the affected calendar fails while triage owners for other calendars continue to update; valid fallbacks should remain unchanged.