fix checking state and return current state with responses

[ad-astra-video]

Fix to send back 503 response if current_client is set.

Also updates to send back current_state in response

[eliteprox]

This change looks good to me. Tested solution resolves the primary issue of interrupting an active stream with a start request.

There is an existing defect that make the status response inaccurate though. This is a universal issue with even /stream/status endpoint. We can address it in a follow-up PR https://github.com/livepeer/pytrickle/tree/fix-get-pipeline-state

{
    "status": "error",
    "message": "Pipeline is still in use",
    "current_state": {
        "status": "OK",
        "state": "OK",
        "error_message": null,
        "pipeline_ready": true,
        "active_streams": 1,
        "startup_complete": true,
        "pipeline_state": "IDLE",
        "additional_info": {}
    }
}