Clean up empty folders if a clone fails immediately.

[olsen232]

Behaviour is unchanged if the clone succeeds enough to populate or partially populate the new directory - data is left where it is (but the repo is not "activated").

Related links:

https://github.com/koordinates/kart/issues/963

Checklist:

• [x] Have you reviewed your own change?
• [x] Have you included test(s)?
• [x] Have you updated the changelog?