This to me gets to the fundamental nature of testing. If errors are part of your test suite then they're happy path too. It's just as much a part of your API su

This to me gets to the fundamental nature of testing. If errors are part of your test suite then they're happy path too. It's just as much a part of your API surface as any other return value. But if you don't have (or want) callers that depend on your specific behavior in error situations then I wouldn't bother testing them. It's my contract that I can and will change the behavior arbitrarily and you can't be mad at me for it.