Suggest --list-subs when no matching subtitle format found

[NSExceptional]

DO NOT REMOVE OR SKIP THE ISSUE TEMPLATE

• I understand that I will be blocked if I intentionally remove or skip any mandatory* field

Checklist

• I'm requesting a feature unrelated to a specific site
• I've looked through the README
• I've verified that I have updated yt-dlp to nightly or master (update instructions)
• I've searched known issues and the bugtracker for similar issues including closed ones. DO NOT post duplicates
• I've read the guidelines for opening an issue

Provide a description that is worded well enough to be understood

When you specify a subtitle format and it happens to be unavailable, the following is printed to stderr:

WARNING: No subtitle format found matching "srt" for language en-US, using ass

It would be nice if the warning could be updated to mention --list-subs for subtitle format errors.

Provide verbose output that clearly demonstrates the problem

• Run your yt-dlp command with -vU flag added (yt-dlp -vU <your command line>)
• If using API, add 'verbose': True to YoutubeDL params instead
• Copy the WHOLE output (starting with [debug] Command-line config) and insert it below

Complete Verbose Output

N/A

[wenbang24]

@NSExceptional is that the kind of prompt you were going for?

[NSExceptional]

@DaPotato69 yes :) thanks