Improve test suite

While the overall test coverage is at 90% at the time of writing, which isn't bad, it could be better. Moreover, a lot of this coverage is due to regression tests, which, while better than not having the coverage at all, can give a misplaced sense of security. We could do with some more targeted tests, in particular for various error cases. I don't mind whether they're written as traditional unit tests or with the patched examples infrastructure we have, as long as they're targeted.

We shouldn't necessarily do all of these sub-issues during this engagement, as this is a lower priority task than many others, but I think they're worth exploring, as the model is now very complex and much of the functionality is only tested via regression tests, whose output is pretty opaque.