Fix a message format error in migration cleanup

In some situations, the volume migration failed. After the cleanup
begin, code was trying to format a string value as an int,
this caused "copy_error = True" not being invoked, resulted
in UnboundLocalError exception.

Change-Id: I445394cc06fe221804901e8014d60e870f0fe2f7
Closes-Bug: #1290317
Merge "Fix a message format error in migration cleanup"