A pure-Haskell SSH server library

#8ssh drops connection without error message on unsupported key type (or: support ed25519)

I tried to connect to darcsden with an ed25519 key and it of course failed, which I don't mind (though such support would be nice).

What was a bit problematic was the failure mode, which was not to give an error about an unsupported keytype or the like, but to just drop the connection with no message (including when running ssh directly in verbose mode). It would be nice to fix this, so that the unsupported key type is more immediately diagnosable.