pybind/mgr/devicehealth: skip legacy objects that cannot be loaded

pybind/mgr/devicehealth: skip legacy objects that cannot be loaded

Log looks like after test:

    2023-12-21T16:09:28.804+0000 7fbe7fd86700  0 [devicehealth DEBUG root] loading object ABC_DEADB33F_FA
    2023-12-21T16:09:28.805+0000 7fbe7fd86700  0 [devicehealth DEBUG root] object rados.Object(ioctx=<rados.Ioctx object at 0x7fbeee0c4668>,key=ABC_DEADB33F_FA,nspace=--default--,locator=None) does not exist because it is deleted in HEAD
    2023-12-21T16:09:28.805+0000 7fbe7fd86700  0 [devicehealth DEBUG root] finished reading legacy pool, complete = True

Credit to Greg Farnum for postulating the cause.

Fixes: https://tracker.ceph.com/issues/63882
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>