fix: Fail loudly if we attempt to perform a setupless load on an extension module that isn't actually an extension

[silasary]

What type of pull request is this?

• [x] Non-breaking code change
• [ ] Breaking code change
• [ ] Documentation change/addition
• [ ] Tests change

Description

The implementation of #671 introduced a bug where it was impossible to test whether a module actually contains an extension, as it would happily find 0 extensions and not throw an error.

This PR causes the client to correctly throw an exception if there is no extension to be loaded.

Changes

Checklist

• [x] I've formatted my code with Black
• [x] I've ensured my code works on Python 3.10.x
• [x] I've tested my code