Wrong format error when trying to import a csv into cms without a slug (cms-import plugin)

[KlotzJesse]

Describe the bug
When you import a csv that should create new entries, without providing a slug, it won't auto generate anymore but rather errors with wrong format. In the console the error states that it expects a slug to exist.

To Reproduce
Steps to reproduce the behavior:

1. Open the csv import plugin
2. Add a new entry without providing a slug column
3. Import
4. Red error, about malformatted csv, without hints to the slug problem

Expected behavior
I would expect that the import plugin will again auto generate the slugs and won't force expect slugs generated by hand for new items, or at least state this error directly in the message, because all errors of that type wont state which malformatted error got catched, therefore not possible to debug for normal users.