services/horizon: Add clarity to error when setting BUCKET_DIR_PATH with captive core

[mollykarcher]

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

Changing the error message returned when BUCKET_DIR_PATH is set when using captive core, just so that it's clear what corrective action needs to be taken.

Why

Partner engineering raised this when working through an issue with Kraken, the error message returned is confusing and doesn't indicate to the end-user the best way to resolve

Invalid captive core toml file could not unmarshal captive core toml: setting BUCKET_DIR_PATH is disallowed, it can cause clashes between instances

[leighmcculloch]

👏🏻