mgr/rbd_support: avoid wedging the task queue if pool is removed

mgr/rbd_support: avoid wedging the task queue if pool is removed

rados.ObjectNotFound exception handler was referencing ioctx variable
which is assigned only if the pool exists and rados.open_ioctx() call
succeeds.  This lead to a fatal error

  mgr[rbd_support] Failed to locate pool mypool
  mgr[rbd_support] execute_task: [errno 2] error opening pool 'b'mypool''
  mgr[rbd_support] Fatal runtime error: local variable 'ioctx' referenced before assignment

and wedged the task queue.  No other commands were processed until
ceph-mgr daemon restart.

Fixes: https://tracker.ceph.com/issues/52932
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>