cephfs: make CephFSError type unexported
cephfs: make CephFSError type unexported

In order to avoid external dependencies on implementation details,
this change replaces CephFSError with the unexported cephFSError. In case
some application really needs access to the integer value, it can use
the pattern
  var errno interface{ Errno() int }
  if errors.As(err, errno) { ... errno.Errno() ... }

Signed-off-by: Sven Anderson <dd860110a62b19214c4ee03aec0abffecb4e86b8@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph