Check validation shortcut

        :param: manifest_id (string) id received in :method:`validate_manifest`
        :returns:
            * True if manifest was valid
            * None if manifest wasn't checked yet
            * validation dict if not valid

def is_manifest_valid(self, manifest_id):
        """Check validation shortcut

        :param: manifest_id (string) id received in :method:`validate_manifest`
        :returns:
            * True if manifest was valid
            * None if manifest wasn't checked yet
            * validation dict if not valid
        """
        response = self.get_manifest_validation_result(manifest_id)
        if response.status_code != 200:
            raise Exception(response.status_code)
        content = json.loads(response.content)
        if not content['processed']:
            return None
        if content['valid']:
            return True
        return content['validation']