Handle 400 response codes from nicknym
nickserver!10 (closed) will make nickserver respond with 400 - client error on invalid requests.
I believe that https://0xacab.org/leap/bitmask-dev/blob/master/src/leap/bitmask/keymanager/nicknym.py#L138 would have to handle these.
Maybe it would be best to handle all 4xx and 5xx codes there?
I'm happy to see if i can come up with a mister for it. @meskio is it the right direction though?