Pool_mgm: Catch NotFound exception correctly

Pool mgm was not handling this exception correctly, so when a retry failed
because a node didn't exist it would stay on the list of nodes to be retried.

This caused an error on every retry (whilst not a critical problem it is
annoying for monitoring).

Change-Id: I6445b754f9a3f1e16d7578430673d885d13b9aa9