Through some debugging and development, I've hit situations in which an

expected model signature could be missing, causing an error far enough

in the process to make the situation hard to debug.
To address this, this change adds some enhanced signature checking in

MockModel, MockMeta, and the base ModelMutator.
The mock support are just assertion checks that stops logic from getting

too far if something goes very wrong.
The ModelMutator checks are more comprehensive. There's no longer an

assumption that a model signature can be returned. It checks, and if one

cannot be found, a suitable exception is raised so that it can be

handled.
While here, a fix to a previous exception has been added, fixing a crash

due to a lack of missing exception message.
Unit tests are in place to ensure the exceptions are raised.

Unit tests pass.
Verified this resulted in a much more reasonable error in the sort of

pathological condition I hit.