Add safetensors to model not found error msg for default use_safetensors value

[davidgxue]

What does this PR do?

Fixes #30601

• See details in the issue linked
• TL;DR: Most people load using .from_pretrained() without explicitly specifying use_safetensors arg. This use_safetensors arg will default to None if not specified AND if system has safetensors library installed. When a model is being loaded and no modeling file is found and use_safetensors is defauled to None, model.safetensors does not get mentioned as part of the error message that lists out all the model files formats it has attempted to load, leading to confusion that safetensors may not be supported or should not be used.
• There are error messages specifically made for loading safetensors but they only cover the explicitly True case. The idea here is that the comprehensive error message should list all model types when use_safetensors is defaulted to None.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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[amyeroberts]

Thanks for digging into this and improving the error messages!