verify behaviour if port discovery fails

[Samuel Allan]

In this section: https://opendev.org/openstack/charm-octavia/src/commit/41faa0023072e572fb7d9c7a1f4614fba4631c18/src/reactive/octavia_handlers.py#L196-L201

```
        except api_crud.APIUnavailable as e:
            ch_core.hookenv.log('Neutron API not available yet, deferring '
                                'port discovery. ("{}")'
                                .format(e),
                                level=ch_core.hookenv.DEBUG)
            return

```

The log message suggests it's going to try again later, but it's unclear if/when it is actually retried. This error should be bubbled up to the user, and probably within the context of the configure-resources action.