Displayed error message for already used channel name (#24860)

* WIP

* Displayed existing channel name error

* refactoring

server/channels/app/channel.go

@@ -697,6 +697,9 @@ func (a *App) UpdateChannel(c request.CTX, channel *model.Channel) (*model.Chann
 		var invErr *store.ErrInvalidInput
 		switch {
 		case errors.As(err, &invErr):
+			if invErr.Entity == "Channel" && invErr.Field == "Name" {
+				return nil, model.NewAppError("UpdateChannel", store.ChannelExistsError, nil, "", http.StatusBadRequest).Wrap(err)
+			}
 			return nil, model.NewAppError("UpdateChannel", "app.channel.update.bad_id", nil, "", http.StatusBadRequest).Wrap(err)
 		case errors.As(err, &appErr):
 			return nil, appErr