Migrate GetChannels method from ChannelStore to return error interface (#14711)

* Migrate GetChannels method from ChannelStore to return error interface

* Fix testing

* Changed error type: ErrInvalidInput -> ErrNotFound

* Added note about error migrations

* Fix en.json

* Fix i18n

Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>

app/channel.go

@@ -1551,7 +1551,18 @@ func (a *App) GetChannelByNameForTeamName(channelName, teamName string, includeD
 }
 
 func (a *App) GetChannelsForUser(teamId string, userId string, includeDeleted bool) (*model.ChannelList, *model.AppError) {
-	return a.Srv().Store.Channel().GetChannels(teamId, userId, includeDeleted)
+	list, err := a.Srv().Store.Channel().GetChannels(teamId, userId, includeDeleted)
+	if err != nil {
+		var nfErr *store.ErrNotFound
+		switch {
+		case errors.As(err, &nfErr):
+			return nil, model.NewAppError("GetChannelsForUser", "app.channel.get_channels.not_found.app_error", nil, nfErr.Error(), http.StatusNotFound)
+		default:
+			return nil, model.NewAppError("GetChannelsForUser", "app.channel.get_channels.get.app_error", nil, err.Error(), http.StatusInternalServerError)
+		}
+	}
+
+	return list, nil
 }
 
 func (a *App) GetAllChannels(page, perPage int, opts model.ChannelSearchOpts) (*model.ChannelListWithTeamData, *model.AppError) {