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**Description**

Related to the discussion here: https://github.com/apache/airflow/pull/8553#discussion_r416136954

Allow user to define expected terminal state. e.g. `JOB_STATE_CANCELLED` could be not treated as failure but expected terminal state.


<!-- A short description of your feature -->


"""
    Start a Templated Cloud Dataflow job; the parameters of the operation will be passed to the job.
    .. seealso::
        For more information on how to use this operator, take a look at the guide:
        :ref:`howto/operator:DataflowTemplatedJobStartOperator`
    :param template: The reference to the Dataflow template.
    :param job_name: The 'jobName' to use when executing the Dataflow template
        (templated).
    :param options: Map of job runtime environment options.
        It will update environment argument if passed.
        .. seealso::
            For more information on possible configurations, look at the API documentation
            `https://cloud.google.com/dataflow/pipelines/specifying-exec-params
            <https://cloud.google.com/dataflow/docs/reference/rest/v1b3/RuntimeEnvironment>`__
    :param dataflow_default_options: Map of default job environment options.
    :param parameters: Map of job specific parameters for the template.
    :param project_id: Optional, the Google Cloud project ID in which to start a job.
        If set to None or missing, the default project_id from the Google Cloud connection is used.
    :param location: Job location.
    :param gcp_conn_id: The connection ID to use connecting to Google Cloud.
    :param poll_sleep: The time in seconds to sleep between polling Google
        Cloud Platform for the dataflow job status while the job is in the
        JOB_STATE_RUNNING state.
    :param impersonation_chain: Optional service account to impersonate using short-term
        credentials, or chained list of accounts required to get the access_token
        of the last account in the list, which will be impersonated in the request.