Fix "Unknown error" on submit when there are validation messages

[bjosttveit]

Description

Validation issues returned by process/next no longer causes unknown error. Instead, they are shown to the user in the error report.

Additionally, we do not look for task validations in regular backend validations anymore, as it turns out task validation does not happen there anyway.

These changes were lifted from the multiple-datamodels branch, as this issue was already fixed there.

Related Issue(s)

• closes #{issue number}

Verification/QA

• Manual functionality testing
  • [x] I have tested these changes manually
  • [ ] Creator of the original issue (or service owner) has been contacted for manual testing (or will be contacted when released in alpha)
  • [ ] No testing done/necessary
• Automated tests
  • [ ] Unit test(s) have been added/updated
  • [x] Cypress E2E test(s) have been added/updated
  • [ ] No automatic tests are needed here (no functional changes/additions)
  • [ ] I want someone to help me make some tests
• UU/WCAG (follow these guidelines until we have our own)
  • [ ] I have tested with a screen reader/keyboard navigation/automated wcag validator
  • [x] No testing done/necessary (no DOM/visual changes)
  • [ ] I want someone to help me perform accessibility testing
• User documentation @ altinn-studio-docs
  • [ ] Has been added/updated
  • [x] No functionality has been changed/added, so no documentation is needed
  • [ ] I will do that later/have created an issue
• Support in Altinn Studio
  • [ ] Issue(s) created for support in Studio
  • [x] This change/feature does not require any changes to Altinn Studio
• Sprint board
  • [x] The original issue (or this PR itself) has been added to the Team Apps project and to the current sprint board
  • [ ] I don't have permissions to do that, please help me out
• Labels
  • [x] I have added a kind/* label to this PR for proper release notes grouping
  • [ ] I don't have permissions to add labels, please help me out

[sonarcloud[bot]]

Quality Gate failed

Failed conditions
62.2% Coverage on New Code (required ≥ 65%)
47.83% Condition Coverage on New Code (required ≥ 65%)

See analysis details on SonarCloud