Viewing validations of subform-instances

### Description

Currently, validating a subform, one text shows in the main form:

(here: "form_filler.error_validation_inside_subform" is overwritten).

Depending on how many instances of the subform that needs to be corrected, it is desired to show a customized validation text.

Example: to show a text aimed at the subform instance, e.g. "Du må fylle ut regnskapsskjema for konsernregnskapet" in this scenario:

We have in total 3 cases where a validation message can show up for the subform component: both instances, or one of the instances (either selskapsregnskap or konsernregnskap).

Today's case, visualising which instance based on the pink background color #F8E4E4, makes users dependent on being able to identify the color contrast (related to WCAG AA).

### Additional Information

_No response_

Start by locating the subform-instance validation rendering and how form_filler.error_validation_inside_subform is selected or overwritten. Define the behavior for both instances and for either individual instance, with text that identifies the affected instance without relying on color; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.