Fatal error when using unknown formatter

The proposed fix / MR does not provide a fatal error; it just shows a warning, but it still allows the user to save the form unmodified (so does not force correcting this mistake before saving).

This happens to be the easiest, because I couldn't make form errors to work immediately. (I don't know if that's because of the table structure, tabledrag.js or whatever.)

But also, I like it this way. If you have other things to change in this form, it's still possible to do those without affecting the missing formatter IDs, and fix things later (by e.g. enabling a missing module).

I reviewed the code and approved the PR.

I tested it locally and it works as it should.

Moving the ticket to RTBC.

Automatically closed - issue fixed for 2 weeks with no activity.