Grouper Provisioner fails if a group already exists in Grouper

The Grouper Provisioner will throw an error when asked to provision memberships into a group that already exists in Grouper. Normally the Grouper Provisioner assumes it has complete control over the base stem into which it is configured to provision groups and it tracks the groups it creates and provisions, but in some circumstances it is helpful if the plugin can just proceed with provisioning into a group that already exists.