Fix Core update rollback: delay image cleanup and fix missing rollback path

[agners]

Proposed change

Fix two issues with the Core update rollback flow:

1. Old image cleaned up too early: The previous version's Docker
   image was removed inside _update() before the post-update health
   checks (frontend loaded and accessible). If the health checks failed
   and a rollback was triggered, the old image had to be re-downloaded.
   Moved the cleanup to after health checks pass.

2. Missing rollback when get_config fails: When get_config()
   raised HomeAssistantError after the update, the code set
   error_state = True but immediately returned, skipping the rollback
   block. The other health check failure paths correctly fall through to
   the rollback logic; this was the only one that didn't.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to the supervisor)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:
• Link to cli pull request:
• Link to client library pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Ruff (ruff format supervisor tests)
• Tests have been added to verify that the new code works.

If API endpoints or add-on configuration are added/changed:

• Documentation added/updated for developers.home-assistant.io
• CLI updated (if necessary)
• Client library updated (if necessary)