Check errors for GetGroupMetaData

When a request for GroupMetadata fails, WhatsApp sends an JSON object
containing a status field: `{"status":500}`. In my tests, it sends the
codes 401, 404 and 500 which might have the meaning of the HTTP status
codes. At least, we don't have data to update the portal and should stop
doing anything.

whatsapp-ext/whatsapp.go

@@ -61,6 +61,8 @@ type GroupInfo struct {
 
 	GroupCreated int64 `json:"creation"`
 
+	Status int16 `json:"status"`
+
 	Participants []struct {
 		JID          string `json:"id"`
 		IsAdmin      bool   `json:"isAdmin"`