Fix REST status code in case of failed request

GridRestResponse should obtain proper 'not-OK' status on error. Often, GridRestResponse#getSuccessStatus() is `GridRestResponse#STATUS_SUCCESS` even if the response is actually an error.

Probably, we should revise GridRestResponse#setError(String). It should set/check an error status like GridRestResponse(int status, @Nullable String err)

A test attached.