test/libcephfs: Test rename error cases

Make sure that rename fails with the ENOENT
if the source path doesn't exist.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
client: Fix rename returning ENOENT for dest

Introduced by fc80c1dc6ee315ae5e039986602ffadba46cb43b,
the client should _not_ fail if the lookup for the
destination path on rename returns ENOENT.

The previous code also did not check that the lookup
returned ENOENT or success.  We add the check and fail
if we get any other errors.

Fixes #4517.
Signed-off-by: Sam Lang <sam.lang@inktank.com>