fix: return error on non-ErrNotExist stat failures in Tar.Sync()

Previously, Sync() only checked for fs.ErrNotExist when classifying
paths into copy vs delete. Non-NotExist stat errors (e.g. EACCES,
EIO) caused the condition to be false, falling through to the else
clause which incorrectly treated the path as copyable. This masked
real errors and led to cryptic failures downstream.

Restructure the condition into a three-way branch:
- err == nil → copy
- ErrNotExist → delete
- other errors → return immediately with context

This follows the pattern already used by entriesForPath() in the
same file.

Fixes #13654

Signed-off-by: Lidang Jiang <lidangjiang@gmail.com>
Signed-off-by: Lidang-Jiang <lidangjiang@gmail.com>