private static void defFail(String id, String val, String type, Exception e) {
        throw new IllegalArgumentException("Invalid value '" + val + "' for " + type + " '" + id + "': " +
                                           Exceptions.toMessageString(e));
    }
Returns the root ConfigDefinition, might be this.
config/src/main/java/com/yahoo/vespa/config/ConfigDefinition.java