Additional error handle for some special cases

* Return err when the read config file has an actual error
* Skip the readInstance call if the response code is not 200. We have a special case that when using our discovery service (based on Eureka's API) that does not provide the same GET API, therefore the call for readInstance will always crash due to dereferencing a nil pointer. We want to simply skip this for our use case. It should not impact the normal behavior when using the Eureka API.