err on invalid feature flag

src/config.rs

@@ -782,7 +782,7 @@ fn validate_config(cfg: &ConfigItems) -> Result<(), Error> {
         &["autofill-overlay", "autofill-v2", "browser-fileless-import", "fido2-vault-credentials"];
     for flag in parse_experimental_client_feature_flags(&cfg.experimental_client_feature_flags).keys() {
         if !KNOWN_FLAGS.contains(&flag.as_str()) {
-            warn!("The experimental client feature flag {flag:?} is unrecognized. Please ensure the feature flag is spelled correctly and that it is supported in this version.");
+            err!("The experimental client feature flag {flag:?} is unrecognized. Please ensure the feature flag is spelled correctly and that it is supported in this version.");
         }
     }