lxd/images: Skip update if image cannot be found
lxd/images: Skip update if image cannot be found

This fixes #8589.

Signed-off-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>

atxwebs/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,atxwebs/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,lxc/lxd