[Building Sakai] Merge request: Kernel API change (KNL-601)

[David Horwitz]

Hi All,

This refers to https://jira.sakaiproject.org/browse/KNL-601 and the
related provider updates in https://jira.sakaiproject.org/browse/SAK-19821

KNL-601 adds a new method the the groupProvider to check that a group
exists. Prior to this change there was no way via the interfaces to
distinguish between an empty group and a invalid groupId. At UCT this is
currently giving us problems as lecturers try to setup courses ahead of
the start of term next month.

Impact of merging: All deployers who have custom group providers will
need to add 1 method to their group provider implementation.

Any objections?

D