locking: Clarify message for removed locks

Just being told that a lock doesn't exist can be confusing. One case
were this happens is when a job (e.g. instance modify) waits for a job
removing the instance (e.g. export with remove).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>