API Rate Limit message should be in JSON

When the rate-limit is exceeded, the API returns a plain-text body that says:

"Rate limit for this resource has been exceeded"

Which is not valid JSON. Because the rest of the API is JSON, it makes sense to return the rate-limit error message in JSON format. Something like this:

{ "error": "Rate limit for this resource has been exceeded" }

would make it easier to handle errors in an automated fashion