By putting the wrong error-type enum value in a ScanKexinitsResult, I
accidentally caused nonsense messages of the form
Selected key exchange algorithm "foo,bar,baz" does not correspond to any supported algorithm
where "foo,bar,baz" is the full comma-separated list sent by the
server, so it's not even _an_ algorithm as the message suggests.
Now the message is the one it should have been all along:
Couldn't agree a key exchange algorithm (available: foo,bar,baz)
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