fix: temporary file leak in atomic_write_file

[anim001k]

Description

Fixed a logical error in the atomic_write_file function that caused temporary files to remain on disk when write_fn failed. The function now properly cleans up temporary files before returning errors, preventing file system pollution.

Changes

• Replaced map_err chain with match statement for proper error handling
• Added temporary file cleanup on write_fn failure
• Improved code comments for better maintainability

Problem

The original code would leave .tmp files on disk when write_fn returned an error, leading to:

• Resource leaks
• File system clutter
• Potential confusion for users

Solution

Now when write_fn fails, the function:

1. Removes the temporary file
2. Returns the original error wrapped in FsPathError::Write

[mattsse]

I see, that makes sense