lxd/db/images: Switch to api.StatusErrorf in GetImage
lxd/db/images: Switch to api.StatusErrorf in GetImage

And other error improvements.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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