Raise an error when invalid guids are set

[#77647762]

Signed-off-by: Mike Dalessio <mdalessio@pivotal.io>
Setting or removing invalid guids now raises an exception.

- added test coverage to vcap_relations_spec
- and to model_controller_spec
- note that there are no API docs changes, despite the checksum update.

[#77647762]

Signed-off-by: Mike Dalessio <mdalessio@pivotal.io>
Merge branch 'invalid_guid_handling'

[#77647762]