clippy: possibly unexpected user experience with `cap-lints=warn` and user-defined `--deny` flags

@buchgr noticed on https://github.com/bazelbuild/rules_rust/pull/2451#issuecomment-1941114502 that the current behavior to cap-lints when combined with user-defined --deny flags might be unexpected for users, as those will be down-graded to warnings:
>However, I'd expect the rules to error if a an unsupported deny lint is specified, rather than silently ignore it.

Also related to https://github.com/bazelbuild/rules_rust/pull/1264.