fix: use json response for errors in /api/ routes

[pi0]

🔗 Linked issue

resolves #951

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

To avoid breaking changes, this PR uses json response again for routes starting with /api

PS: This is temporary. We might find better solutions to show pretty error pages for API later. Also notes in #951 (comment)

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.