`eprintln!` warnings are not (easily) testable

[jieyouxu]

Remark (not for this PR): hm, these eprintln!s feel problematic to me in terms of being able to actually test them. We probably need to not directly use eprintln! and instead use some other custom interceptor which allows promoting warnings -> errors when testing [or such] 🤔

Originally posted by @jieyouxu in #6651 (comment)

There are a few instances, the one I noticed was

rustfmt/src/config/config_type.rs

Lines 437 to 444 in 4929253

	if override_value > max_width {
	eprintln!(
	"`{0}` cannot have a value that exceeds `max_width`. \
	`{0}` will be set to the same value as `max_width`",
	config_key,
	);
	return max_width;
	}