Map NFS3ERR_NOTEMPTY to ENOTEMPTY

Map NFS3ERR_NOTEMPTY to ENOTEMPTY rather than EEXIST.  POSIX allows
either EEXIST or ENOTEMPTY for rmdir on a non-empty directory but
ENOTEMPTY is more explicit and in line with Linux, OS X, FreeBSD and
OpenBSD's behavior.

Signed-off-by: Ross Lagerwall <rosslagerwall@gmail.com>