load-fragment: ignore ENOTDIR/EACCES errors (#3510)

load-fragment: ignore ENOTDIR/EACCES errors (#3510)

If for whatever reason the file system is "corrupted", we want
to be resilient and ignore the error, as long as we can load the units
from a different place.

Arch bug https://bugs.archlinux.org/task/49547.

A user had an ntfs symlink (essentially a file) instead of a directory after
restoring from backup. We should just ignore that like we would treat a missing
directory, for general resiliency.

We should treat permission errors similarly. For example an unreadable
/usr/local/lib directory would prevent (user) instances of systemd from
loading any units. It seems better to continue.