Rename all errors to be prefixed with Err.

bot.go

@@ -64,7 +64,7 @@ func (bot *BotAPI) MakeRequest(endpoint string, params url.Values) (APIResponse,
 	defer resp.Body.Close()
 
 	if resp.StatusCode == http.StatusForbidden {
-		return APIResponse{}, errors.New(APIForbidden)
+		return APIResponse{}, errors.New(ErrAPIForbidden)
 	}
 
 	bytes, err := ioutil.ReadAll(resp.Body)

configs.go

@@ -30,8 +30,8 @@ const (
 
 // API errors
 const (
-	// APIForbidden happens when a token is bad
-	APIForbidden = "forbidden"
+	// ErrAPIForbidden happens when a token is bad
+	ErrAPIForbidden = "forbidden"
 )
 
 // Constant values for ParseMode in MessageConfig