libzip: fix extraction of folders without zip entry

Some zip archives may not have a zip entry for a folder, but only the
entries for the files within that folder.

The Part generates a fake `Archive::Entry` in such cases, which gets
added the to list of entries passed to the `ExtractJob`.

This is properly handled by unzip (which prints a "caution: filename not
matched" warning for the virtual entry) and 7z. However, the libzip
plugin would abort the whole extraction when it couldn't stat the
missing entry.

This patch just prints a warning when it detects a folder that cannot be
stat'd (due to the NOENT error), but the extraction will continue as normal.

BUG: 394542
FIXED-IN: 18.04.3