Rollback db actions when connection invalidated

[samhotep]

Done

Added an error handler for sqlalchemy.exc.PendingRollbackError which occurs when a connection object is invalidated

QA

• The branch has been deployed to staging. Open https://staging.ubuntu.com/security/cves and https://staging.ubuntu.com/security/notices, and check that there are no issues.

Issue / Card

Fixes intermittent issue where a timed out transaction (normally on the security/notices.json endpoint) invalidates the current connection and requires a rollback before reconnecting.