Handle mlab-ns errors in update-bouncer.py script
As reported by Jordan McCarthy the update-bouncer.py script currently does not support cases in which mlab-ns reports an oonibackend instance, but does not include the .onion address. In these cases these backends should be considered offline and not added to the bouncer.
We should add a check for this in the above script.
The ones that are giving errors will have the "tool_extra" key set to
"tool_extra": "NRPE: Unable to read output"