Allow for easier troubleshooting of protocol version exchange

[drieseng]

When the protocol version exchange fails, we currently do not provide any information of what data we actually received from the server. We also do not distinguish between a server that immediately closed the connection (after having received the client SSH identification string), and a server that sends something else instead of a SSH identification string.

There are two issues to fix:

• Use a separate exception message to indicate that a server did not send any data at all.
• Provide detailed information on the actual data received when the server did not send a valid SSH identification string.