Don't die when a LoRA is a broken symlink

[akx]

Description

• a simple description of what you're trying to accomplish: when a LoRA is a broken symlink, it can be enumerated fine, but attempting to read it (for hashing) will crash.
• which issues it fixes, if any: Fixes [Bug]: FileNotFoundError on broken symlinks #11098

Screenshots/videos:

*** Failed to load LoRA moop from stable-diffusion-webui/models/Lora/moop.safetensors
    Traceback (most recent call last):
      File "stable-diffusion-webui/extensions-builtin/Lora/lora.py", line 452, in list_available_loras
        entry = LoraOnDisk(name, filename)
      File "stable-diffusion-webui/extensions-builtin/Lora/lora.py", line 101, in __init__
        hashes.sha256_from_cache(self.filename, "lora/" + self.name, use_addnet_hash=self.is_safetensors) or
      File "stable-diffusion-webui/modules/hashes.py", line 51, in sha256_from_cache
        ondisk_mtime = os.path.getmtime(filename)
      File "lib/python3.10/genericpath.py", line 55, in getmtime
        return os.stat(filename).st_mtime
    FileNotFoundError: [Errno 2] No such file or directory: 'stable-diffusion-webui/models/Lora/moop.safetensors'

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests