librbd: fix create existence checking

librbd: fix create existence checking

cda9e516b8bb09b8846814cc8d4ee2879a53b2d5 made us return 0 when the
image already existed, causing copy to erroneosly ignore an existing
image. Separate the case where we know the image exists from being
unable to tell whether it exists because of e.g. an authentication
problem.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>