Fix `make graph-entities` command

A recent PR more fully implemented `OneToManyField`s. This is good. However,
that PR also broke the graph-entities script. This commit inserts some
type-checking into `scripts.graph-entities`, thus fixing the breakage which
occurred when `OneToManyField`s were improved.

Also:

* Create an `Errata` entity. The entity is read-only.
* Visually distinguish between entities which are factories and entities which
  are not. This makes it easier to determine how much progress has been made in
  creating factories (and, therefore, tests).