Don't charge clients when server-side errors occur

[philippgille]

For example when the web service is configured wrong, the client shouldn't pay.

Payments should not be restricted to the 2xx range though, bad requests should probably be paid, too.

Also, probably not everyone wants this, so make it configurable, maybe up to the point to configure which status codes to charge for, and maybe even how many Satoshis. E.g.: 100 Satoshis for a bad request, because the client is keeping the server a little busy, but 1000 Satoshis for a request that leads to a call of a paid third party API.