Description of problem:


It happens only if orphaned content hasn't been fully migrated with the first run.

To reproduce:

 * have orphaned content in pulp 2
 * run migration, stop it before the content migration is done (ensure that at 
 * least one of the orphaned content units was pre-migrated but not migrated.)
 * clean orphans in pulp2
 * run migration again

{
    "child_tasks": [],
    "created_resources": [
        "/pulp/api/v3/task-groups/f6d728d2-61ea-49f2-b461-f8e0baf33b9b/"
    ],
    "error": {
        "description": "[Errno 2] No such file or directory: '/var/lib/pulp/content/units/iso/d7/269cf0f9afe9445d5e31c82cf13be3b17f75af57312a1638659ac592c221fc/1.iso'",
        "traceback": "  File \"/usr/local/lib/pulp/lib64/python3.6/site-packages/rq/worker.py\", line 936, in perform_job\n    rv = job.perform()\n  File \"/usr/local/lib/pulp/lib64/python3.6/site-packages/rq/job.py\", line 684, in perform\n    self._result = self._execute()\n  File \"/usr/local/lib/pulp/lib64/python3.6/site-packages/rq/job.py\", line 690, in _execute\n    return self.func(*self.args, **self.kwargs)\n  File \"/home/vagrant/devel/pulp-2to3-migration/pulp_2to3_migration/app/tasks/migrate.py\", line 141, in migrate_from_pulp2\n    migrate_content(plan, skip_corrupted=skip_corrupted)\n  File \"/home/vagrant/devel/pulp-2to3-migration/pulp_2to3_migration/app/migration.py\", line 47, in migrate_content\n    plugin.migrator.migrate_content_to_pulp3(skip_corrupted=skip_corrupted)\n  File \"/home/vagrant/devel/pulp-2to3-migration/pulp_2to3_migration/app/plugin/iso/migrator.py\", line 64, in migrate_content_to_pulp3\n    loop.run_until_complete(dm.create())\n  File \"/usr/lib64/python3.6/asyncio/base_events.py\", line 484, in run_until_complete\n    return future.result()\n  File \"/home/vagrant/devel/pulp-2to3-migration/pulp_2to3_migration/app/plugin/content.py\", line 89, in create\n    await pipeline\n  File \"/home/vagrant/devel/pulpcore/pulpcore/plugin/stages/api.py\", line 225, in create_pipeline\n    await asyncio.gather(*futures)\n  File \"/home/vagrant/devel/pulpcore/pulpcore/plugin/stages/api.py\", line 43, in __call__\n    await self.run()\n  File \"/home/vagrant/devel/pulp-2to3-migration/pulp_2to3_migration/app/plugin/content.py\", line 178, in run\n    self.migrate_to_pulp3(cmodel, ctype)\n  File \"/home/vagrant/devel/pulp-2to3-migration/pulp_2to3_migration/app/plugin/content.py\", line 378, in migrate_to_pulp3\n    downloaded=pulp2content.downloaded\n  File \"/home/vagrant/devel/pulp-2to3-migration/pulp_2to3_migration/app/plugin/content.py\", line 128, in create_artifact\n    expected_size=expected_size)\n  File \"/home/vagrant/devel/pulpcore/pulpcore/app/models/content.py\", line 277, in init_and_validate\n    with open(file, \"rb\") as f:\n"
    },
    "finished_at": "2020-11-23T11:12:54.853991Z",
    "name": "pulp_2to3_migration.app.tasks.migrate.migrate_from_pulp2",
    "parent_task": null,
    "progress_reports": [
        {
            "code": "creating.repositories",
            "done": 0,
            "message": "Creating repositories in Pulp 3",
            "state": "completed",
            "suffix": null,
            "total": 0
        },
        {
            "code": "migrating.importers",
            "done": 0,
            "message": "Migrating importers to Pulp 3",
            "state": "completed",
            "suffix": null,
            "total": 0
        },
        {
            "code": "migrating.content",
            "done": 0,
            "message": "Migrating content to Pulp 3",
            "state": "failed",
            "suffix": null,
            "total": 0
        },
        {
            "code": "migrating.iso.content",
            "done": 13,
            "message": "Migrating iso content to Pulp 3 iso",
            "state": "failed",
            "suffix": null,
            "total": 276
        },
        {
            "code": "processing.repositories",
            "done": 4,
            "message": "Processing Pulp 2 repositories, importers, distributors",
            "state": "completed",
            "suffix": null,
            "total": 4
        },
        {
            "code": "premigrating.content.general",
            "done": 0,
            "message": "Pre-migrating Pulp 2 ISO content (general info)",
            "state": "completed",
            "suffix": null,
            "total": 0
        },
        {
            "code": "premigrating.content.detail",
            "done": 0,
            "message": "Pre-migrating Pulp 2 ISO content (detail info)",
            "state": "completed",
            "suffix": null,
            "total": 0
        }
    ],
    "pulp_created": "2020-11-23T11:12:54.394083Z",
    "pulp_href": "/pulp/api/v3/tasks/e1a1a18e-bebe-4f74-bde8-804432964765/",
    "reserved_resources_record": [
        "pulp_2to3_migration"
    ],
    "started_at": "2020-11-23T11:12:54.538645Z",
    "state": "failed",
    "task_group": "/pulp/api/v3/task-groups/f6d728d2-61ea-49f2-b461-f8e0baf33b9b/",
    "worker": "/pulp/api/v3/workers/74ba07cd-4830-4527-9c70-452f16fe2de5/"
}

Version-Release number of selected component (if applicable):
pulp-2to3-migration 0.6.0