raise error with non-existence image prompts

[Gasoonjia]

Currently image_prompt variable in the error message doesn't exist, blocking error msg to be printed out:

Traceback (most recent call last):
  File "/home/gasoonjia/torchchat/torchchat.py", line 88, in <module>
    generate_main(args)
  File "/home/gasoonjia/torchchat/torchchat/generate.py", line 1249, in main
    generator_args = GeneratorArgs.from_args(args)
  File "/home/gasoonjia/torchchat/torchchat/generate.py", line 184, in from_args
    raise RuntimeError(f"Image prompt {image_prompt} does not exist")
NameError: name 'image_prompt' is not defined. Did you mean: 'image_prompts'?

After this PR:

(torchchat) [gasoonjia ~/torchchat (image-prompt-bug)]$ python torchchat.py generate llama3.2-11B --prompt "What's in this image?" --image-prompt assets/NO_EXIST_IMG.jpg
Traceback (most recent call last):
  File "/home/gasoonjia/torchchat/torchchat.py", line 88, in <module>
    generate_main(args)
  File "/home/gasoonjia/torchchat/torchchat/generate.py", line 1225, in main
    generator_args = GeneratorArgs.from_args(args)
  File "/home/gasoonjia/torchchat/torchchat/generate.py", line 188, in from_args
    raise RuntimeError(
RuntimeError: Image prompt ['assets/NO_EXIST_IMG.jpg'] does not exist

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