services/horizon: Gracefully handle incorrect assets in GET /offers

[2opremio]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• 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

• 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

Fixes #2466

Gracefully handle incorrect assets in the query parameters of GET /offers.

Before this change, a panic happened and a 500 HTTP error was returned.

Why

A 500 is incorrect and not helpful for the end user.

Known limitations

N/A

[abuiles]

can you add a regression test with query params that would caught this error? Also we should have caught that it was invalid in the custom validators for each part of the asset, that is assetType or asset

[2opremio]

@abuiles PTAL

[2opremio]

Also we should have caught that it was invalid in the custom validators for each part of the asset, that is assetType or asset

I would rather address this separately. But I will do it here if you think I should.