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>

store/timer_layer.go

@@ -840,7 +840,7 @@ func (s *TimerLayerChannelStore) GetChannelUnread(channelId string, userId strin
 	return resultVar0, resultVar1
 }
 
-func (s *TimerLayerChannelStore) GetChannels(teamId string, userId string, includeDeleted bool) (*model.ChannelList, *model.AppError) {
+func (s *TimerLayerChannelStore) GetChannels(teamId string, userId string, includeDeleted bool) (*model.ChannelList, error) {
 	start := timemodule.Now()
 
 	resultVar0, resultVar1 := s.ChannelStore.GetChannels(teamId, userId, includeDeleted)