Cannot set validation errors on Confirmation task

### Description of the bug

I would assume the metod ValidateTask in ValidationHandler is made to run validation on any task, not just the one concering filling out the form. However, when adding a modelError while on the confirmation task the form crashes (Ukjendt feil) with the following 409 network error:
`Cannot complete/close current task TaskBekreftelse. The data element(s) assigned to the task are not valid!`

### Steps To Reproduce

1. Use an app that contains a confirmation step.
2. On the confirmation step, trigger a validation error in `ValidateTask` with `validationResults.AddModelError` on a hidden field.
3. When running the app, observe that it crashes upon clicking 'confirm' on the confirmation step.

### Additional Information

Checking for e.g. outdated data on confirmation could be a real case particularly when the roles enabled to fill out a form and the ones enabled to confirm it differs.

Start with ValidateTask in ValidationHandler and reproduce the confirmation-step case using validationResults.AddModelError on a hidden field. Trace how that result is handled when clicking confirm, then verify the chosen behavior with an app containing a confirmation step; done means the confirmation flow no longer crashes with the reported 409 error and validation remains meaningful.

Written by the indexing model from the issue text.