Don't attempt resubmission if we get a HTTP 400 response

[Nick-Adams-AU]

I have identified an issue with Librato if I attempt to submit a metric where a composite metric by the same name already exists. This results in Librato returning a HTTP 400 error due to the name collision. statsd-librato identifies the non-HTTP-200 response and prints an error:

statsd 2015/07/03 09:05:02 unable to submit measurements: 400 Bad Request: {"errors":{"params":{"type":["'metric.restapi.requesttime' is a GAUGE, but was submitted as different type"]}},"request_time":1435878302}

Rather than statsd-librato moving on from the error, it appears to constantly attempt resubmission of the invalid data at each interval. New stats submissions don't occur as the erroneous data is constantly resubmitted until it is accepted - which of course it never will.

Temporary errors like HTTP 500/503..etc, should be reattempted but permanent errors such as HTTP 4xx should be logged, droped and we should move on.