WIP: Clarify missing associated type error message

This MR is to address #21100 - To give a clearer indication on what's wrong and how to fix undeclared associated types.

I ended up veering off from the original error message wording a bit, but I think it's an improvement. I'm unsure if I've done anything glaringly wrong here, so feel free to let me know and I'll be happy to fix things.

Also, I updated some of the tests to accommodate for change in error message wording, but it still appears there are some failures occurring with the tests. I'm not sure if that was due to this change or if they were already there.