[YouTube Downloader] Improve error message

[LitoMore]

Description

Resolves #15768
Resolves #16584
Fixes #16795

• Improve error message
• Fix URL validator while link has no protocol prefix
• Replace execa with nano-spawn
• Adjust import orders
• Fix yt-dlp from preferences

The previous error message always shows the Invalid YouTube URL even on some yt-dlp execution errors. I've exposed the actual yt-dlp error message to users for a better expression.

I also updated the error message Invalid YouTube URL to Invalid URL to prepare for the support of other URLs as #16570 (comment) mentioned.

This also fixed three issues #15768 #16584 #16795 that user cannot select their specific yt-dlp from preferences.

Checklist

• I read the extension guidelines
• I read the documentation about publishing
• I ran npm run build and tested this distribution build in Raycast
• I checked that files in the assets folder are used by the extension itself
• I checked that assets used by the README are placed outside of the metadata folder