Give app developer the option to change texts on the "you can't start this app" page

## Description
The page displaying validation messages from the `InstantiationHandler.RunInstantiationValidation()` method in an app currently only display standard texts in addition to the generated messages.

The page must support all text resource features like language, variables and markdown. It should be possible to replace the standard texts with custom texts and it should be possible for the method to return text resource keys. (This last bit might already be supported, must be tested/verified.)

It could also be that the page needs a design refresh. See screenshot.

⚠️ _NOTES_ ⚠️
This issue is implicitly solved by introducing the new texts microservice.

## Screenshots
The current version of the page as of the creation of this issue.

## Considerations

### Ops requirements

## Acceptance criteria

## Specification tasks

- [ ] Development tasks are defined

## Development tasks

## Test

## Definition of done

- [ ] Documentation (docs.altinn.studio) is updated (if relevant)
- [ ] Technical documentation
- [ ] User documentation
- [ ] QA
- [ ] Manual test is complete (if relevant)
- [ ] Automated test is implemented (if relevant)
- [ ] All tasks in this userstory are closed (i.e. remaining tasks are moved to other user stories or marked obsolete)

Start by reading the page that displays validation messages from InstantiationHandler.RunInstantiationValidation(). Verify whether the method already accepts text resource keys, then trace how standard texts and generated messages are rendered. Done means custom language, variable, and markdown-enabled texts can replace the standard texts, with the behavior verified through appropriate tests; the issue notes this may be superseded by the new texts microservice.

Written by the indexing model from the issue text.