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/storetest/mocks/ChannelStore.go

@@ -560,7 +560,7 @@ func (_m *ChannelStore) GetChannelUnread(channelId string, userId string) (*mode
 }
 
 // GetChannels provides a mock function with given fields: teamId, userId, includeDeleted
-func (_m *ChannelStore) GetChannels(teamId string, userId string, includeDeleted bool) (*model.ChannelList, *model.AppError) {
+func (_m *ChannelStore) GetChannels(teamId string, userId string, includeDeleted bool) (*model.ChannelList, error) {
 	ret := _m.Called(teamId, userId, includeDeleted)
 
 	var r0 *model.ChannelList
@@ -572,13 +572,11 @@ func (_m *ChannelStore) GetChannels(teamId string, userId string, includeDeleted
 		}
 	}
 
-	var r1 *model.AppError
-	if rf, ok := ret.Get(1).(func(string, string, bool) *model.AppError); ok {
+	var r1 error
+	if rf, ok := ret.Get(1).(func(string, string, bool) error); ok {
 		r1 = rf(teamId, userId, includeDeleted)
 	} else {
-		if ret.Get(1) != nil {
-			r1 = ret.Get(1).(*model.AppError)
-		}
+		r1 = ret.Error(1)
 	}
 
 	return r0, r1