Database Upgrade; Can now recover partially upgraded databases

When a database upgrade fails, typically the whole process needed to be repeated.

With MS Access databases, this was not generally too much of an issue since the original database was never modified.

With SQL Server databases however, this meant restoring the original database and retrying.

A new 'Checkpoint' mechanism has now been implemented meaning that partially upgraded database can now be upgraded again to continue the upgrade.

WARNING: This is not necessarily fool proof so ALWAYS BACKUP YOUR DATABASE prior to performing an upgrade.