private static String getMessage(Throwable t) {
        String message = t.getMessage();
        if (t.getCause() == null) {
            if (message == null) return t.getClass().getSimpleName();
        }
        return message;
    }
Returns a useful message from *this* exception, or null if there is nothing useful to return
vespalog/src/main/java/com/yahoo/log/VespaFormatter.java