What does this PR do?

When loading a LoRA adapter, so far, there was only a warning when there were unexpected keys in the checkpoint. Now, there is also a warning when there are missing keys.

This change is consistent with https://github.com/huggingface/peft/pull/2118 in PEFT and the planned PR https://github.com/huggingface/diffusers/pull/9622 in diffusers.

Apart from this change, the error message for unexpected keys was slightly altered for consistency (it should be more readable now). Also, besides adding a test for the missing keys warning, a test for unexpected keys warning was also added, as it was missing so far.

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [x] Did you write any new necessary tests?