SSHD port forwarding. Missing SSH_MSG_CHANNEL_OPEN_FAILURE message if failed to open connection

Hi we are faceing issue using port forwarding feature.

An ssh client is requesting to open a remote connection through Apache sshd server.

In case connection to remote server fails, there some cases where sshd do not send back SSH_MSG_CHANNEL_OPEN_FAILURE.

The problem is in org.apache.sshd.common.session.helpers.AbstractConnectionService class 

The offending code seems to be from line 642 to 654: in case Exception is null, no SSH_MSG_CHANNEL_OPEN_FAILURE is sent back to client.

We'll try to post a pull request for this issue