Allow sys.argv and args in LightningCLI

[Erotemic]

What does this PR do?

Currently this just changes an error into a warning. An alternative would be to let the user specify a flag that changed the error into a warning, but my thought is that might just be more clutter than what it's worth, raising the warning might be good enough.

Fixes #16807

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[carmocca]

The failing test needs to be updated