worker: don't try to remove opstate for deletes

Only the copy operation creates an op state. If the object does not
exist, there is no op state to delete.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Include exception information in warnings

This will provide more detail when an error occurs without requiring
logging at the debug level.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>