DEV: Barf more usefully on empty string for docker status.
DEV: Barf more usefully on empty string for docker status.

ibab/dockerspawner,anaderi/dockerspawner,jlamcanopy/dockerspawner,jhamrick/dockerspawner,betatim/dockerspawner,Xarthisius/dockerspawner,anaderi/dockerspawner,jhamrick/dockerspawner,betatim/dockerspawner,quantopian/dockerspawner,minrk/dockerspawner,Xarthisius/dockerspawner,bigzachattack/dockerspawner,minrk/dockerspawner,jupyter/dockerspawner,ibab/dockerspawner,jlamcanopy/dockerspawner,quantopian/dockerspawner,Fokko/dockerspawner,bigzachattack/dockerspawner,jupyter/dockerspawner,Fokko/dockerspawner