[NXCM-5392] Extract member validation method to be able to override it in subclasses
[NXCM-5392] Add InvalidGroupingException constructor for a prebuilt reason
[NXCM-5392] Do no allow P2 composite groups as members of P2 legacy groups
Sent the actual reason to client in case of an invalid member
P2 composite group should have an P2CompositeGroupRepository facet
Merge pull request #64 from sonatype/NXCM-5392-block-composite-on-p2-legacy

REVIEW: [NXCM-5392] Block P2 composite groups as member of P2 legacy group