Display error message when locking a vpm fails due to downburst errors.
When doing a lock-many, do not lock any of the vpms when downburst errors
occur.   Do not display message on unlock with downburst error, because
slot is still freed (and can be locked by someone else).

Fix: 5957
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Merge pull request #32 from ceph/wip-fix5957-wusui

Display error message when locking a vpm fails due to downburst errors.