Make behaviour less surprising for CSVs that fail to upload via "Create a codelist" form

Why are we doing this?

From @rw251 in #2705, when a CSV fails to be processed:

the CSV file is no longer "uploaded". I therefore did this one time:

• upload a csv with headers
• don't click "Yes" to headers
• click "Create"
• return to create page with the "your csv is invalid" type message
• click "Yes" to headers
• click "Create"
• end up at the builder page for an empty codelist because without a selected CSV it just creates an empty codelist
• I was confused

How will we know when it's done?

When this behaviour is changed to be less surprising.

What are we doing?

If possible, one of the following might address this:

• persisting the CSV on failed upload, so that the CSV isn't cleared
• validating that a CSV is present when clicking "Yes" or "No" on the header option is selected, and then perhaps failing with something like "you made a CSV header choice, but didn't provide a CSV"

Or there may be some alternative approach that makes the behaviour more obvious to the user.

Reproduce the failure flow in the Create a codelist form: upload a CSV, make an incorrect header choice, and submit. Trace how the form handles the failed upload and the later header choice; done means the CSV is retained or the user receives clear validation, and an empty codelist cannot be created unexpectedly.