Throw an error when a transcode for a given assetId is not empty.

[matvp91]

Describe the solution you'd like
We allow people to set an assetId in the transcode API call. We should schedule the job, and let Artisan check whether the assetId leads to a previously transcoded asset and throw an error. We should only transcode assetId's that have not been transcoded before.

Additionally, introduce a force flag on the transcode API endpoint to instruct Artisan to delete the folder with the assetId, no matter what. And retranscode it.