mon: Monitor: clearer output on error during attempt to convert store

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #121 from ceph/wip-4448

mon: Monitor: clearer output on error during attempt to convert store

Reviewed-by: Sage Weil <sage@inktank.com>
Merge git://github.com/ceph/ceph