Improve POST URL status error (#250)

* Improve POST URL status error

* Mirror the connection error string

internal/symbolizer/symbolizer.go

 	}
 	defer resp.Body.Close()
 	if resp.StatusCode != http.StatusOK {
-		return nil, statusCodeError(resp)
+		return nil, fmt.Errorf("http post %s: %v", source, statusCodeError(resp))
 	}
 	return ioutil.ReadAll(resp.Body)
 }