Raise exception if there is no schema on the resource

[mbeilin]

We should modify ckanext-aircan to error if there is no schema on the resource.

Acceptance

• [x] The exception raised if there is no schema on the resource.

Tasks

• [x] Raise exception in submit action in case there is no schema on the resource. The Exception should be as follows:

  This resource has no schema so cannot be imported into the DataStore. Please add a Table Schema in the resource schema attribute. See XXX section in AirCan docs [on github] for more

[mbeilin]

Sample request (without schema)

{
  "package_id": "my-first-dataset",
  "url":  "gs://europe-west2-aircan-airflow-1f8e0bdc-bucket/dags/aircan/dags/r3.csv",
  "description": "This is the best resource ever!" ,
  "name": "Hi65"
}

will produce the following response:

{
    "help": "http://ckan:5000/api/3/action/help_show?name=resource_create",
    "success": false,
    "error": {
        "message": "Not found: {'Schema Not Found': 'This resource has no schema so cannot be imported into the DataStore. Please add a Table Schema in the resource schema attribute. See `Airflow instance on Google Composer` section in AirCan docs [https://github.com/datopian/ckanext-aircan#airflow-instance-on-google-composer] for more.'}",
        "__type": "Not Found Error"
    }
}

[mbeilin]

DONE - closed with PR - https://github.com/datopian/ckanext-aircan/pull/18