Metric error for fallback-success

[Tyler Bischel]

https://github.com/afex/hystrix-go/blob/master/hystrix/metrics.go#L57

Not sure that this was intended, but it looks like a fallback-success event would result in the error incrementing.

[keith]

this is intended behavior.

execution of the fallback happens in response to an error. so even if the fallback succeeds, the error occurred and we track that.