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.   Made error messages more accurate, and removed a destroy_if_vm call
because the destroy was alreadly called in unlock.  Changed some print
messages to be log.info displays.

Fix: 5957
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Merge pull request #37 from ceph/wip-fix5957-wusui

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