Ensure that authoring_api cannot create a corrupted data state

[kdmccormick]

There are ways in which you can call the authoring_api that leads the DB to be broken. For example, before we fixed it, you were able to:

• Create a container like a Unit
• Create a new ContainerVersion, but NOT a new UnitVersion
• Now, app code will see the ContainerVersion and assume it has a .unit_version, causing a 500
• There's no way out of this state without modifying MySQL yourself

AC: Scan through authoring_api and ensure that any "invalid" operation raises an exception rather than putting the DB in a bad state.