Disable object_storage for export task

This MR disables object_storage for export rake task, the same way we are already doing for the import rake task.

When both object_storage and :direct_upload are enabled, we always use REMOTE storage.

This is causing our Gitlab::ImportExport::AfterExportStrategies::MoveFileStrategy to fail, since exported project archive is not found on the file system.

Also, in this MR, an error message is now logged/printed in case there was an error during export.

Closes: #216018 (closed)