MM-57391: improve error message (#26630)

The error reporte when moving channels and failing has been improved to show
that the problem was the repeated name on the team.
The error message has been unified with MM-53756

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/channel.go

@@ -622,12 +622,12 @@ func (a *App) UpdateChannel(c request.CTX, channel *model.Channel) (*model.Chann
 	_, err := a.Srv().Store().Channel().Update(c, channel)
 	if err != nil {
 		var appErr *model.AppError
+		var uniqueConstraintErr *store.ErrUniqueConstraint
 		var invErr *store.ErrInvalidInput
 		switch {
+		case errors.As(err, &uniqueConstraintErr):
+			return nil, model.NewAppError("UpdateChannel", store.ChannelExistsError, nil, "", http.StatusBadRequest).Wrap(err)
 		case errors.As(err, &invErr):
-			if invErr.Entity == "Channel" && invErr.Field == "Name" {
-				return nil, model.NewAppError("UpdateChannel", store.ChannelExistsError, nil, "", http.StatusBadRequest).Wrap(err)
-			}
 			return nil, model.NewAppError("UpdateChannel", "app.channel.update.bad_id", nil, "", http.StatusBadRequest).Wrap(err)
 		case errors.As(err, &appErr):
 			return nil, appErr
@@ -3149,10 +3149,13 @@ func (a *App) MoveChannel(c request.CTX, team *model.Team, channel *model.Channe
 	channel.TeamId = team.Id
 	if _, err := a.Srv().Store().Channel().Update(c, channel); err != nil {
 		var appErr *model.AppError
+		var uniqueConstraintErr *store.ErrUniqueConstraint
 		var invErr *store.ErrInvalidInput
 		switch {
 		case errors.As(err, &invErr):
 			return model.NewAppError("MoveChannel", "app.channel.update.bad_id", nil, "", http.StatusBadRequest).Wrap(err)
+		case errors.As(err, &uniqueConstraintErr):
+			return model.NewAppError("MoveChannel", store.ChannelExistsError, nil, "", http.StatusBadRequest).Wrap(err)
 		case errors.As(err, &appErr):
 			return appErr
 		default: