new error codes, code refactor, added getMessages function

model/model.go

@@ -25,3 +25,11 @@ type Contact struct {
 type ContactSuggestion struct {
 	Username string `json:"username"`
 }
+
+type Message struct {
+	MessageID     int    `json:"message_id"`
+	Message       string `json:"message"`
+	SentAt        string `json:"sent_at"`
+	Sender        string `json:"sender"`
+	AttachmentUrl string `json:"attachment_url"`
+}