use error 429 for rate limiting

proposed fix for #69

BEFORE
"error": {"code": 402,"details": ["This is a metered resource, number of requests have exceeded the rate limit interval."], "message": "Unable to proxy request for requested resource."}}

AFTER
"error": {"code": 429,"details": ["This is a metered resource, number of requests have exceeded the rate limit interval."], "message": "Error 429 - Too Many Requests"}}