Error in harvest_source_list after deleting a harvest source

[bzar]

1. Create a harvest source
2. Delete the harvest source
3. Call harvest_source_list action with return_last_job_status=False
4. Everything works, the deleted source is included
5. Call harvest_source_list action with return_last_job_status=True
6. ckan.logic.NotAuthorized: User None not authorized to read harvest source f00babab-41a8-4167-98f6-1ebe295c23bb from harvest_source_show_status

I'd provide a PR but I'm not exactly sure how the issue should be resolved. For example:

1. Skip sources that fail authorization
2. Skip deleted sources
3. Provide a placeholder status for deleted sources

[FuhuXia]

I vote for:

2. Skip deleted sources