bitcoind errors are swallowed

[devrandom]

HttpClient.read_response bails out without reading the body if ! status.is_ok(). However, normally servers use a non-OK status code when returning an error description in the body, so this swallows all error information and just returns a NotFound with a generic message to the caller. In particular bitcoind returns a 500 error, even while returning a meaningful JSON error (?!).

Instead, we should read the body and propagate a more detailed error to the API user (all the way out of RpcClient.call_method). We should also log this when invoking call_method from inside the lightning crate.