feat: Improve error messages for config and cache directory creation

[PreistlyPython]

Summary

This PR improves error messages when GitUI fails to create config or cache directories due to permission issues or other filesystem errors. Instead of showing a generic error, users now receive clear, contextual information about which directory GitUI was trying to create.

Problem

Fixes #2683 - Better error message if config (and/or HOME) directory is not writeable

Currently, when GitUI fails to create config directories due to permission issues, users see a generic error message:

Error: Permission denied (os error 13)

This provides no context about what GitUI was trying to do, making it difficult for users to understand and resolve the issue.

Solution

The fix adds contextual error messages that clearly indicate:

1. What GitUI was trying to do (create config/cache directory)
2. Which specific directory path was involved
3. The underlying system error

Before this change:

Error: Permission denied (os error 13)

After this change:

Failed to create config directory: /home/user/.config/gitui
Error: Permission denied (os error 13)

Failed to create cache directory: /home/user/.cache/gitui  
Error: Permission denied (os error 13)

Implementation

The changes are minimal and focused:

In process_cmdline() (line 52-58):

fs::create_dir_all(&confpath).map_err(|e| {
    anyhow!(
        "Failed to create config directory: {}\nError: {}",
        confpath.display(),
        e
    )
})?;

In get_app_cache_path() (line 163-169):

fs::create_dir_all(&path).map_err(|e| {
    anyhow!(
        "Failed to create cache directory: {}\nError: {}",
        path.display(),
        e
    )
})?;

Testing

• ✅ Code compiles without errors
• ✅ No breaking changes to existing functionality
• ✅ Error messages provide clear context for debugging
• ✅ Maintains the same error propagation behavior

Benefits

1. Better user experience - Users can immediately identify the problematic directory
2. Easier troubleshooting - No need to use strace or similar tools to debug permission issues
3. Consistent error reporting - Follows the pattern of providing context with system errors
4. Non-breaking change - Error handling behavior remains the same, only the message is improved

This addresses the exact issue described in #2683 where users had to resort to using strace to identify which directory was causing permission problems.

🤖 Generated with Claude Code