The ${FAILED_TESTS} token takes too much chars to put it in mail subject, so it only useful in mail body.
It would be much better if provide tokens to get failed unit test count and total unit test count, so i can configure the mail subject like this:

$PROJECT_NAME $BUILD_NUMBER - $BUILD_STATUS - ${FAILED_TESTS_COUNT} of ${ALL_TESTS_COUNT} tests failed

BTW:
I konw there is a solution based on an unofficial patch(http://joesondow.blogspot.com/2009/10/fancy-hudson-email.html). However, i think to provide Content-Token is much simpler, and i don't like unofficial patches.