ChannelStore migration Part 3 (#15504)

* Migration finished

* Change error var name

* Fix imports

* Fix tests

* Merge with master

* Doing some suggestions

* More suggestions

* Fix i18n

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>

migrations/advanced_permissions_phase_2.go

@@ -87,7 +87,7 @@ func (worker *Worker) runAdvancedPermissionsPhase2Migration(lastDone string) (bo
 	} else if progress.CurrentTable == "ChannelMembers" {
 		// Run a ChannelMembers migration batch.
 		if data, err := worker.srv.Store.Channel().MigrateChannelMembers(progress.LastChannelId, progress.LastUserId); err != nil {
-			return false, progress.ToJson(), err
+			return false, progress.ToJson(), model.NewAppError("MigrationsWorker.runAdvancedPermissionsPhase2Migration", "app.channel.migrate_channel_members.select.app_error", nil, err.Error(), http.StatusInternalServerError)
 		} else {
 			if data == nil {
 				// We haven't progressed. That means we've reached the end of this final stage of the migration.