exp/lighthorizon/actions: use standard Problem model on API error responses

[sreuland]

PR Checklist

PR Structure

• [x] This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• [x] This PR avoids mixing refactoring changes with feature changes (split into two PRs otherwise).
• [x] This PR's title starts with name of package that is most changed in the PR, ex. services/friendbot, or all or doc if the changes are broad or impact many packages.

Thoroughness

• [x] This PR adds tests for the most critical parts of the new functionality or fixes.
• [ ] I've updated any docs (developer docs, .md files, etc... affected by this change). Take a look in the docs folder for a given service, like this one.

Release planning

• [ ] I've updated the relevant CHANGELOG (here for Horizon) if needed with deprecations, added features, breaking changes, and DB schema changes.
• [ ] I've decided if this PR requires a new major/minor version according to semver, or if it's mainly a patch change. The PR is targeted at the next release branch if it's not a patch change.

What

Apply the standard Problem json model on API error responses.

Why

to be consistent with the same Problem json model that classic horizon applies on API error responses for cmp tool usage.

Known limitations