Changed all methods returning successful boolean to just throw exceptions, which is generally what they were doing on failure anyway.
Changed Relationships to require concrete Entities, rather than just assuming they tied back to an existing Entity.
Fixed corresponding "breaks" to UI from refactor.
Fixed bug causing exceptions when adding new Relationships via UI.
Fixed bug with RelationshipPanes incorrectly setting Relationship name rather than Entity on Entity Update event.
Added a lot of tests. Reached 100% coverage on Entity.