Migrate Get/GetFromMaster methods from ChannelStore to return error interface (#14688)

* Advances

* Migration finished

* Rename err to normalized error

* fix imports

* Renamed key

* Renamed key

* Suggestions

* Fix i18n

* Fix tests

Co-authored-by: Jesús Espino <jespinog@gmail.com>
Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>

store/sqlstore/group_store.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	sq "github.com/Masterminds/squirrel"
+	"github.com/pkg/errors"
 
 	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/store"
@@ -480,7 +481,13 @@ func (s *SqlGroupStore) CreateGroupSyncable(groupSyncable *model.GroupSyncable)
 		insertErr = s.GetMaster().Insert(groupSyncableToGroupTeam(groupSyncable))
 	case model.GroupSyncableTypeChannel:
 		if _, err := s.Channel().Get(groupSyncable.SyncableId, false); err != nil {
-			return nil, err
+			var nfErr *store.ErrNotFound
+			switch {
+			case errors.As(err, &nfErr):
+				return nil, model.NewAppError("CreateGroupSyncable", "store.sql_channel.get.existing.app_error", nil, nfErr.Error(), http.StatusNotFound)
+			default:
+				return nil, model.NewAppError("CreateGroupSyncable", "store.sql_channel.get.find.app_error", nil, err.Error(), http.StatusInternalServerError)
+			}
 		}
 
 		insertErr = s.GetMaster().Insert(groupSyncableToGroupChannel(groupSyncable))