Ability to catch/log/handle errors

Hey there,

thanks for your great library. We've started using it for hundreds of SSH tunnels and it works great so far!

Since in our case the tunnels come from hundreds of different networks, network blips are quite common and we started noticing a few recurring error messages:

- `ssh: unexpected packet in response to channel open: ` happened about 1.3k times in about 24h
- `ssh: rejected: connect failed (Connection refused)` happened about 250 times in the same time span.

I understand that these are messages from the golang ssh library (`ssh/mux.go` and `ssh/connection.go`), but there is currently no way to handle/catch there errors in your library (see https://github.com/gliderlabs/ssh/blob/master/tcpip.go#L146-L151), since the errors are just printed to STDOUT.

Would it be possible to add a way to deal with these situations? If anything, just to be able to associate the error message with a specific SSH connection.