Cirrus reports error of preview url not set

[jaredlockhart]

It looks like not setting the preview collection url causes cirrus to spit out some spurious error logging. Since the preview mode is entirely optional, we should make sure that if preview url is not set that

1. we don't attempt to fetch the preview collection
2. calling in with nimbus_preview=true raises a 400 error and a note that preview mode isn't configured for this cirrus instance
3. no additional spurious error logging

ERROR:cirrus.experiment_recipes:Failed to fetch recipes: Invalid URL '': No scheme supplied. Perhaps you meant https://?
ERROR:cirrus.main:Failed to fetch preview recipes: Invalid URL '': No scheme supplied. Perhaps you meant https://?
ERROR:apscheduler.executors.default:Job "fetch_schedule_recipes (trigger: interval[0:00:10], next run at: 2024-09-17 15:55:57 UTC)" raised an exception
Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/apscheduler/executors/base_py3.py", line 30, in run_coroutine_job
    retval = await job.func(*job.args, **job.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/cirrus/cirrus/main.py", line 287, in fetch_schedule_recipes
    schedule_retry()
  File "/cirrus/cirrus/main.py", line 291, in schedule_retry
    app.state.scheduler.add_job(
  File "/usr/local/lib/python3.11/site-packages/apscheduler/schedulers/base.py", line 434, in add_job
    'trigger': self._create_trigger(trigger, trigger_args),
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/apscheduler/schedulers/base.py", line 935, in _create_trigger
    return self._create_plugin_instance('trigger', trigger, trigger_args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/apscheduler/schedulers/base.py", line 920, in _create_plugin_instance
    return plugin_cls(**constructor_kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: IntervalTrigger.__init__() got an unexpected keyword argument 'max_retries'

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