Add exhaustive checks for errors in contracts

[maurolacy]

Some contracts (like subkeys) have error conditions / cases that are not being triggered / checked in unit tests. Let's add those for completeness.

[ethanfrey]

This is too vague.
Can you please give some concrete examples so we can make PRs to fix them (can be new issues per contract, or adding a checklist to this issue). If they have been since fixed, let's close this.

[maurolacy]

This was useful as an initial task for @hashedone to get used to the framework.

I would say, let's close this, and in any case create specific issues to improve testing, and the test framework / techniques.

[maurolacy]

Closing as too vague / general.