readConfig have error check

config.go

 import (
 	"encoding/base64"
 	"encoding/json"
+	"errors"
 	"io/ioutil"
 	"log"
 )
 
 	//AppSecret is how we identify ourselves.
 	AppSecret string `json:"AppSecret"`
+
+	//AuthTokenSaveTo
+	AuthTokenSaveTo string `json:"AuthTokenSaveTo"`
 }
 
 //APIConfig contains secrets that cannot store in source file
 	log.Printf("read config from %s\r\n", "server_config.json")
 	body, err := ioutil.ReadFile("server_config.json")
 	if err != nil {
+		log.Fatal("Cannot read config from server_config.json")
 		return err
 	}
-	return json.Unmarshal(body, &APIConfig)
+	err = json.Unmarshal(body, &APIConfig)
+	if APIConfig.AppSecret == "" {
+		return errors.New("AppSecret not available")
+	}
+	if APIConfig.Appid == "" {
+		return errors.New("AppId not available")
+	}
+	if APIConfig.AuthTokenSaveTo == "" {
+		return errors.New("where to save accesstoken is unknown")
+	}
+	if APIConfig.EncodingAESKey == "" {
+		return errors.New("unknown AesEncryptionKey")
+	}
+	if APIConfig.Token == "" {
+		return errors.New("hidden shared secret token is empty")
+	}
+	return err
 }
 
 //convert AesEncryptKey into Binary 32bytes