[24.0] Raise exception if collection elements missing during download

[jdavcs]

Fixes #18075

To do:

• Decide what type of exception to raise instead of generic Exception
• Client fix: grey out the download button if the collection isn't populated yet

Is this a good location for raising the exception? I assume if that method is called, the collection must be populated? Or should we let it break and catch the AttributeError instead in the api module?

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

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[mvdbeek]

I'm happy to separate backend and frontend fix if you want, returning an exception here is already a better UX than the 500 users would get.

[jdavcs]

I'm happy to separate backend and frontend fix if you want, returning an exception here is already a better UX than the 500 users would get.

No, thanks, I'm working on it. I'll get it done today.

[mvdbeek]

Thank you!