Any error triggers the circuit breaker

[ruiaraujo]

Due to how https://github.com/zalando-incubator/perron/blob/master/lib/client.ts#L606-L608 looks like, any error thrown can trigger the circuit breaker even if the request was never made.

This increases the impact of bugs in perron or the underlying Node implementation.

My expectation is that the circuit breaker would be notified of error only once the request had been made to the backend.

[grassator]

We could either hardcode this logic or provide a config option with a function that would filter things out. In 0.9.0 more detailed error information was added, so we could probably look at the error constructor to figure out what happened.