Attempt to copy mchirp_source.json does not check if preferred event has mchirp.json

Seeing errors on playground right now where a burst pipeline is preferred, eg S250515j, and a task tries and fails to download mchirp_source.json from the preferred event, but that file doesn't exist. The task fails with

Traceback (most recent call last):
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/celery/app/trace.py", line 477, in trace_task
    R = retval = fun(*args, **kwargs)
                 ^^^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/sentry_sdk/utils.py", line 1811, in runner
    return sentry_patched_function(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/sentry_sdk/integrations/celery/__init__.py", line 415, in _inner
    reraise(*exc_info)
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/sentry_sdk/utils.py", line 1746, in reraise
    raise value
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/sentry_sdk/integrations/celery/__init__.py", line 410, in _inner
    return f(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/celery/app/trace.py", line 760, in __protected_call__
    return self.run(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/celery/app/autoretry.py", line 38, in run
    return task._orig_run(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/gwcelery/tasks/gracedb.py", line 33, in wrapper
    return f(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/gwcelery/tasks/gracedb.py", line 185, in download
    with client.events[graceid].files[filename].get() as f:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/gracedb_sdk/api/files.py", line 22, in get
    raw = self.session.get(self.url, stream=True).raw
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/requests/sessions.py", line 602, in get
    return self.request("GET", url, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/requests_gracedb/file.py", line 97, in request
    return super(SessionFileMixin, self).request(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/requests/sessions.py", line 589, in request
    resp = self.send(prep, **send_kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/requests/sessions.py", line 710, in send
    r = dispatch_hook("response", hooks, r, **kwargs)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/requests/hooks.py", line 30, in dispatch_hook
    _hook_data = hook(hook_data, **kwargs)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/requests_gracedb/errors.py", line 21, in _hook_raise_errors
    response.raise_for_status()
  File "/home/emfollow-playground/.local/lib/python3.11/site-packages/requests/models.py", line 1024, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://gracedb-playground.ligo.org/api/events/G2979309/files/mchirp_source.json