room power-levels should fail gracefully when Synapse not reachable

[JOJ0]

$ synadm room power-levels
ERROR ConnectionError while querying Synapse: HTTPConnectionPool(host='localhost', port=8008): Max retries exceeded with url: /_synapse/admin/v1/rooms?from=0&limit=10 (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x10405aa90>: Failed to establish a new connection: [Errno 61] Connection refused'))
...
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  File "/Users/user/git/synadm/synadm/api.py", line 849, in room_power_levels
    for i, room in enumerate(rooms["rooms"]):
                             ~~~~~^^^^^^^^^
TypeError: 'NoneType' object is not subscriptable