Better error when a bad directory is given for weight merging

[muellerzr]

What does this PR do?

This PR expounds the error messages given when passing in a checkpoint directory, specifying if model or optimizer FSDP directories were found.

Each of these have pytorch_model_fsdp_0 and optimizer_0 for folders, so we can check for their existence and direct the user on what to do.

Fixes https://github.com/huggingface/accelerate/issues/2848

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.
• [ ] Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR.

@SunMarc

[HuggingFaceDocBuilderDev]

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