Fix remaining issues with internal Downloader API

[Jackenmen]

Description of the changes

Fixes regressions from #6706:

• "---" being appended when all cogs failed to install
  • The prefix will also no longer be shown when none of the cogs has an install message, though that was not part of the regression
• Incorrect type hints
• Wrong argument being passed to _update_cogs() when repo fails to update in cog updatetoversion

Have the changes in this PR been tested?

Yes