Make the blacklist validation error slightly clearer

validators.go

@@ -135,7 +135,7 @@ type ValidatorFunc func(context.Context, string) error
 func notInSet(set stringSet) ValidatorFunc {
 	return func(_ context.Context, value string) error {
 		if set.Contains(value) {
-			return errors.New("reserved value")
+			return errors.New("invalid value (blacklisted)")
 		}
 		return nil
 	}