Provide more SSH output if connection fails

[Ma27]

I've seen output like this a few times already

redacted00: Connecting via ssh (1/3)
redacted01: Connecting via ssh (2/3)
redacted10: Connecting via ssh (3/3)
Connection closed by UNKNOWN port 65535
Connection closed by UNKNOWN port 65535
Connection closed by UNKNOWN port 65535
Connection closed by UNKNOWN port 65535
Connection closed by UNKNOWN port 65535
Connection closed by UNKNOWN port 65535
Connection closed by UNKNOWN port 65535
ERROR: Unexpected exception
<traceback>
    execnet.gateway_bootstrap.HostNotFound: redacted00.fcio.net

There are a few causes for that issue:

• the private key is missing a trailing newline and SSH can't load it
• failing hostkey verification
  ...and probably more

The output shown above makes diagnosing this very hard. At the very least I'd like to see the error from SSH.

Additionally

• the backtrace doesn't serve any value here, so it would be cool if it could be hidden in that case.
• often the cause for something like this is on the client-side. Perhaps we could run ssh with increased debug level (or make this configurable) and capture the debug-output silently. Then, if the connection fails, print the output above the error message.