Client does not take proxy errors into account

[hielkehoeve]

The client always assumes it will get an ErrorReport from KeyHub when an error occurs:
eg: https://github.com/topicuskeyhub/go-keyhub/blob/master/groups.go#L85

This will only work if all possible situations will return an error response in this json format. When a proxy returns a gateway timout or the webserver is (re)starting the error response will not match the json format and a json unmarshall error will occur, losing all useful information about what went wrong.