Check that model_save_name exists before trying to load it, to avoid confusing checkpoint error

[TheBloke]

Very simple fix that checks that the model exists before trying to load it.

Without this, if the user passes the wrong folder/file name, they get this error:

ValueError: `checkpoint` should be the path to a file containing a whole state dict, or the index of a sharded checkpoint, or a folder containing a sharded checkpoint, but got /workspace/stable-vicuna-13B-GPTQ/stable-vicuna-13B-GPTQ-4bit1.latest.act-order.safetensors.

I found this quite confusing because it does not clearly indicate that the file was not found at all.

With this PR, the user will now get:

FileNotFoundError: Could not find model at /workspace/stable-vicuna-13B-GPTQ/stable-vicuna-13B-GPTQ-4bit1.latest.act-order.safetensors

[PanQiWei]

Great! This will make error message clearer for users to understand what's actually going wrong.