INT-3305 SFTP Close Session on Error

JIRA: https://jira.springsource.org/browse/INT-3305

Previously, if the channel can't be connected (e.g. if the server
does not support SFTP), the connection to the server remained open.

Close the session in the event of a failure.

Add test with the Apache SSHD server.

__cherry-pick to 3.0.x__