[2.5] Attempting to register a rack without secret (and likely with incorrect secret) causes unhandled error

I attempted to register a rack controller to a region cluster but I forgot to create the secret file. The region logs started showing unhandled errors.

2018-09-11 20:52:53 RegionServer,187,::ffff:10.90.90.5: [info] RegionServer connection lost (HOST:IPv6Address(TCP, '::ffff:192.168.100.1', 5252) PEER:IPv6Address(TCP, '::ffff:10.90.90.5', 51842))
2018-09-11 20:52:53 maasserver.rpc.regionservice: [critical] Rack controller 'None' could not be authenticated; dropping connection.
        Traceback (most recent call last):
        Failure: twisted.protocols.amp.UnhandledCommand: (b'UNHANDLED', 'Unknown Error [node03:pid=6732:cmd=Authenticate:ask=1]')

2018-09-11 20:52:53 maasserver.rpc.regionservice: [info] Rack controller 'None' disconnected.
2018-09-11 20:52:53 RegionServer,189,::ffff:10.90.90.5: [info] RegionServer connection lost (HOST:IPv6Address(TCP, '::ffff:10.90.90.1', 5251) PEER:IPv6Address(TCP, '::ffff:10.90.90.5', 44784))
2018-09-11 20:52:53 maasserver.rpc.regionservice: [critical] Rack controller 'None' could not be authenticated; dropping connection.
        Traceback (most recent call last):
        Failure: twisted.protocols.amp.UnhandledCommand: (b'UNHANDLED', 'Unknown Error [node03:pid=6732:cmd=Authenticate:ask=1]')

To reproduce
------------

1. installed maas-rack-controller
2. configured /etc/maas/rackd.conf
3. sudo service maas-rackd restart

Expected behavior
-----------------

I would have expected that the regiond would not spit unhandled errors, rather, say that the rack controller failed to authenticate due to a missing secret (or incorrect secret).