CLI: crash on misconfigured scheduler class/URL

Sentry Issue: SWH-SCHEDULER-8V

AttributeError: 'NoneType' object has no attribute 'get_task_type'
(5 additional frame(s) were not displayed)
...
  File "click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "swh/scheduler/cli/admin.py", line 67, in runner
    task_type = scheduler.get_task_type(task_type_name)