core/config: Fix ConfigError missing argument if toml is missing

[Holzhaus]

Fixes the following error:

Traceback (most recent call last):
  File "bin/bandit", line 8, in <module>
    sys.exit(main())
  File "lib/python3.10/site-packages/bandit/cli/main.py", line 455, in main
    b_conf = b_config.BanditConfig(config_file=args.config_file)
  File "lib/python3.10/site-packages/bandit/core/config.py", line 45, in __init__
    raise utils.ConfigError(
TypeError: ConfigError.__init__() missing 1 required positional argument: 'config_file'

[ericwb]

LGTM