500 when creating child resources for parents that don't exist

Looks like we don't gracefully handle missing parents in the Craton API, so if someone tries to, say, create a cell for a region that doesn't exist, we get back a 500.

Example:

$ curl -s http://127.0.0.1:8080/v1/regions -H "Content-Type: application/json" -H "X-Auth-Token: demo" -H "X-Auth-User: demo" -H "X-Auth-Project: b9f10eca66ac4c279c139d01e65f96b4" | jq
{
  "links": [...],
  "regions": [
    {
      "created_at": "2017-02-24T20:51:19.000000",
      "id": 1,
      "name": "ORD135",
      "note": null,
      "project_id": "b9f10eca-66ac-4c27-9c13-9d01e65f96b4",
      "updated_at": null
    },
    {
      "created_at": "2017-02-24T20:51:20.000000",
      "id": 2,
      "name": "DFW",
      "note": null,
      "project_id": "b9f10eca-66ac-4c27-9c13-9d01e65f96b4",
      "updated_at": null
    }
  ]
}

Note there are only regions 1 and 2. Now try creating a cell for region 3, which doesn't exist.

$ curl -X POST -s http://127.0.0.1:8080/v1/cells -H "Content-Type: application/json" -H "X-Auth-Token: demo" -H "X-Auth-User: demo" -H "X-Auth-Project: b9f10eca66ac4c279c139d01e65f96b4" -d '{"name": "thomas-cell-1", "region_id": 3}'
{
  "message": "Unknown Error",
  "status": 500
}

Error in API logs:

2017-02-24 20:52:16.609 192 ERROR craton.api.v1.resources.inventory.cells [req-869e5402-ca4a-4571-bec5-641783a5e96b - - - - -] Error during Cells.post: (pymysql.err.IntegrityError) (1452, 'Cannot add or update a child row: a foreign key constraint fails (`craton`.`cells`, CONSTRAINT `cells_ibfk_2` FOREIGN KEY (`region_id`) REFERENCES `regions` (`id`))') [SQL: 'INSERT INTO cells (created_at, updated_at, region_id, project_id, name, note, variable_association_id) VALUES (%(created_at)s, %(updated_at)s, %(region_id)s, %(project_id)s, %(name)s, %(note)s, %(variable_association_id)s)'] [parameters: {'updated_at': None, 'created_at': datetime.datetime(2017, 2, 24, 20, 52, 16, 606973), 'region_id': 3, 'name': 'thomas-cell-1', 'note': None, 'variable_association_id': 32, 'project_id': 'b9f10eca66ac4c279c139d01e65f96b4'}]