c/r: check state before doing a checkpoint/restore
c/r: check state before doing a checkpoint/restore

This would already fail, but with a not-as-good error message. Let's make
the error better.

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>

hallyn/lxc,terceiro/lxc,caglar10ur/lxc,ss1h2a3tw/lxc,ss1h2a3tw/lxc,LynxChaus/lxc,cjwatson/lxc,ysbnim/lxc,aeris/lxc,thmo/lxc,hfuCN/lxc,caglar10ur/lxc,federicobriata/lxc,ysbnim/lxc,GreatFruitOmsk/lxc,hfuCN/lxc,hallyn/lxc,terceiro/lxc,federicobriata/lxc,cjwatson/lxc,GreatFruitOmsk/lxc,LynxChaus/lxc,GreatFruitOmsk/lxc,caglar10ur/lxc,terceiro/lxc,aeris/lxc,federicobriata/lxc,ysbnim/lxc,LynxChaus/lxc,ss1h2a3tw/lxc,thmo/lxc,hfuCN/lxc,thmo/lxc,aeris/lxc,hallyn/lxc,cjwatson/lxc