[concourse]: cleanup repositry directory on failure

[kallisti5]

In the event of an upload failure, the concourse pipelines can leave a "broken" repository for an hrev.

We should likely investigate an on_failure step which cleans up the hrev folder in s3 if the upload fails.
However, in the event that s3 is 100% unavailable, this may not work.

The better solution is hpkgbouncer needs to validate the "completeness" of the hrev repository directory before sending users there. This would likely be accomplished by:

• Examining each repo file
• Validating that all packages exist for the given repo file, and the sha256 checksums are valid
• Adding to RepositoryCache

hpkg bouncer needs a native rust crate that can read hpkr files however to accomplish this.

[kallisti5]

A note for future viewers: The workaround is to kick off a new concourse job to re-build and re-upload the failed hrev repo. (or manually delete the folder from s3)