Bug fixes, mostly around videos that were being wrongly turned away.

• Fixed: some videos were reported as having no captions when they did have them. This was the most common way the add-on failed.
• Fixed: after moving between videos without reloading the page, the language list could still describe the previous video.
• Fixed: retrying after a failed export no longer stalls. It now works on the second attempt.
• Fixed: if you had the transcript panel open yourself, it is left open afterwards instead of being closed for you.
• When an export does fail, the message now explains what actually happened instead of reporting an empty track.