get_scratch_devices(): catch CommandFailedError, log "dev in use"
get_scratch_devices(): catch CommandFailedError, log "dev in use"

Signed-off-by: Dan Mick <b07550071eaa6a9296289c43bbd6c90559196431@inktank.com>
Reviewed-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@inktank.com>

yghannam/teuthology,ktdreyer/teuthology,ivotron/teuthology,robbat2/teuthology,michaelsevilla/teuthology,dmick/teuthology,t-miyamae/teuthology,tchaikov/teuthology,robbat2/teuthology,ivotron/teuthology,t-miyamae/teuthology,ceph/teuthology,dreamhost/teuthology,dmick/teuthology,zhouyuan/teuthology,SUSE/teuthology,caibo2014/teuthology,dreamhost/teuthology,caibo2014/teuthology,tchaikov/teuthology,dmick/teuthology,ceph/teuthology,yghannam/teuthology,ktdreyer/teuthology,michaelsevilla/teuthology,SUSE/teuthology,zhouyuan/teuthology,SUSE/teuthology