Improve UX for setting up transcripts

[eteubert]

• When setting up the asset, vtt is currently under captions but is should be under transcript. And I don't think we need to "Podigee Transcript" format any more
• When there is a vtt asset but the "Episode Transcript Assignment" is empty, there should be a notice suggesting to use the asset
• when there is an error importing transcripts, the error shows as [object Object]

[eigenstil]

• And I don't think we need to "Podigee Transcript" format any more

I think this should remain in the selection because this is actually only plaintext. And some podcast player apps can use this format (e.g. Podcast addict). Therefor it would be better to rename it to "plaintext".