Migrate multiples methods from ChannelStore to return error interface (#14708)

Automatic Merge

app/import_functions.go

@@ -1150,7 +1150,7 @@ func (a *App) getChannelsForPosts(teams map[string]*model.Team, data []*PostImpo
 	for _, postData := range data {
 		team := teams[*postData.Team]
 		if channel, ok := channels[*postData.Channel]; !ok || channel == nil {
-			var err *model.AppError
+			var err error
 			channel, err = a.Srv().Store.Channel().GetByName(team.Id, *postData.Channel, true)
 			if err != nil {
 				return nil, model.NewAppError("BulkImport", "app.import.import_post.channel_not_found.error", map[string]interface{}{"ChannelName": *postData.Channel}, err.Error(), http.StatusBadRequest)