clean P2P protocol shutdown on EOF try 2

Same goal as b18fb1e343 but without
breaking backwards compatability. Just return IO exceptions when running
the P2P protocol, so that git-annex-shell can detect eof and avoid the
ugly message.

This commit was sponsored by Ethan Aubin.

Remote/Helper/Ssh.hs

@@ -343,7 +343,7 @@ runProtoConn a conn@(P2P.OpenConnection (runst, c, _, _)) = do
 		-- When runFullProto fails, the connection is no longer
 		-- usable, so close it.
 		Left e -> do
-			warning $ "Lost connection (" ++ e ++ ")"
+			warning $ "Lost connection (" ++ P2P.describeProtoFailure e ++ ")"
 			conn' <- fst <$> liftIO (closeP2PSshConnection conn)
 			return (conn', Nothing)