Specify Error for Invalid Tests

[DavePearce]

This is important because some invalid tests may eventually fail for other reasons, and will need to be updated. For example, constraints which don't return an expression of type loob will fail eventually for that reason (when currently they are not).

[DavePearce]

This is now reasonably critical because of the addition of a type checker which can be quite restrictive. Specifically, it means that a number of the invalid tests will be failing now for the wrong reasons.