should sprocket warn or fail when an unknown rule is passed to `--except`?

[a-frantz]

Current behavior is essentially ignoring bad values to --except. They are assumed to be valid and never checked. The result of passing an unknown rule to --except is the same as just not specifying the arg at all. Is this desired behavior?

[a-frantz]

Thoughts on this @adthrasher @peterhuene ?

I think my and Peter talked about this back when the --except flag was first added, but I'm not sure we made the right call anymore.

[adthrasher]

I'd probably go with a warning. It's worth noting to the user that a specified rule doesn't exist, but I don't think it's necessarily an error.

[a-frantz]

I'd probably go with a warning. It's worth noting to the user that a specified rule doesn't exist, but I don't think it's necessarily an error.

Should it just be prepended to the list of printed diagnostics? I assume that if we don't consider it an error, we'll continue with the full lint analysis. That would be a special case, as I don't think sprocket currently emits any diagnostics of its own.