Cancel button should not validate the form, errors appear on next modal opened

Patch works for me.

> Hmm, why does the cancel button need to perform a full ajax submit anyway

I don't know if the forms with without JS but there should be provision for that, so we need to handle it in the backend anyway.

Simplified to use the #limit_validation_errors option on the Cancel button.

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