Gracefully handle mtime read exception from cache

[catboxanon]

Description

When a file's mtime is attempted to be read from the cache, but the resultant file does not actually exist (in the case of a symlink), currently this causes the entire Checkpoints/LoRA/etc UI to be forced into Gradio's error state. This fixes that by handling the exception here.

Checklist:

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