If file upload fails from app-frontend the user should be presented with a more instructive error message

## Description
If a file upload fails in app frontend we present a generic error message to the end-user. If we have a concrete reason the file is rejected this error message should be presented to the user.
Related to Altinn/altinn-studio#4383.

## Screenshots

## Considerations
- How to support multiple languages?

## Acceptance criteria
- If we have a concrete reason for rejecting the file this should be returned to app frontend and presented to the user

## Specification tasks

- [ ] Development tasks are defined
- [ ] Test design / decide test need

## Development tasks
> Add tasks here

## Definition of done
Verify that this issue meets [DoD](https://confluence.brreg.no/display/T3KP/Definition+of+Done#DefinitionofDone-DoD%E2%80%93utvikling) (Only for project members) before closing.

- [ ] Documentation is updated (if relevant)
- [ ] Technical documentation (docs.altinn.studio)
- [ ] User documentation (altinn.github.io/docs)
- [ ] 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 tracing the file-upload rejection path in app-frontend and review the related Altinn/altinn-studio#4383 issue; no files or tests are named in the report. Done means concrete rejection reasons reach the frontend and are shown to users, with multiple-language handling and appropriate verification addressed.

Written by the indexing model from the issue text.