added API call error message retrieval (not tested)

todo.tasks

+wechatApiError:
+    ☐ test get JSON error message when api call happened

wechatApiError.go

+package main
+
+import (
+	"encoding/json"
+	"net/http"
+	"strconv"
+)
+
+//WechatError when API call goes wrong, it shows error message
+type WechatError struct {
+	ErrCode int64  `json:"errcode"`
+	ErrMsg  string `json:"errmsg"`
+}
+
+//getWechatAPIError  given an http body,  try to decode json error
+//             caller needs to makesure content type is json
+//return
+//             the error strucutre, on success
+//             err: if json decode encounters problem
+func getWechatAPIError(respBody string) (result WechatError, err error) {
+	err = json.Unmarshal([]byte(respBody), &result)
+	return
+}
+
+//see if a http response is json response
+func isJSON(r *http.Response) bool {
+	contentType := r.Header.Get("Content-Type")
+	contentLen := r.Header.Get("Content-Length")
+	len, _ := strconv.Atoi(contentLen)
+
+	if contentType == "text/plain" || contentType == "text/json" {
+		if len < 10240 { //less than 10K
+			return true
+		}
+	}
+	return false
+}
+
+//To Do: test error message @todo