Use 429, not 500, for tarpit
The tarpit will use a status code of 500 by default, but as we're returning a content of the 429 page anyway, seems minimally useful.
We're also sending a bunch of headers (prefix RateLimit) indicating what the actual limits are and how close the client is to hitting them, so "hiding" that behind a 500 seems pretty pointless.
Edited by Craig Miskell