files: added errors on draft create

[carlinmack]

❤️ Thank you for your contribution!

Description

On initial draft creation, no errors were being forwarded for empty files while on successive saves/updates they do.

Before and after of pressing save draft once (to create it). With my change the files not being there is also shown as an error, as both create and update check for the files

I recommend reviewing by reading the commits in order

• first commit I add a test case that there should be an error on draft create
• second commit I extract assign_files_enabled into a common function as the implementations slightly differed but should be the same
• third commit I extract check_files_exist and add this to the create handler
• fourth commit is responding to christoph's review

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[sakshamarora1]

Do these errors show up in the UI? If so, can you please add a screenshot?
Not sure why the tests for 3.12 are failing as the logs aren't available anymore.

[carlinmack]

just rebased on master so the tests can run again, and added a picture