Missing message is now 404 instead of 500

- Added smtpd.ErrNotExist to make detecting missing message easier
- Return 404 instead of 500 when requesting a non-extistant message
- More REST unit tests

web/mailbox_controller.go

@@ -120,6 +120,10 @@ func MailboxShow(w http.ResponseWriter, req *http.Request, ctx *Context) (err er
 		return fmt.Errorf("MailboxFor('%v'): %v", name, err)
 	}
 	msg, err := mb.GetMessage(id)
+	if err == smtpd.ErrNotExist {
+		http.NotFound(w, req)
+		return nil
+	}
 	if err != nil {
 		return fmt.Errorf("GetMessage() failed: %v", err)
 	}