[24.2] Fix error message when subworkflow input connection missing

[mvdbeek]

Fixes #19511:

There's more we can do:

• Elevate disconnected required inputs to error in the best practice panel
• Show the best practice panel unconditionally
• Check for disconnected inputs on submission

I'll do some of these, but I think this would go beyond a simple bug fix

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

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[jdavcs]

Looks fine.

The failing test fails consistently on 24.2 (passed on 24.1 and dev), so it's not related to this PR.