Project Export should include 'failed' state

Project Export has state that can be one of none/started/after_export/finished. However it does not include failed state. It is not listed in export api either https://docs.gitlab.com/ee/api/project_import_export.html#export-status

So if something goes wrong and export fails, state is not going to properly reflect the reality.

This case should be covered, if Project Export fails, it should be reflected on its state, and clear to the user that something went wrong.