Report the name of the channel for diagnosis.

Reference: ZD-23093
Improving diagnostics for NoClassDefFoundError: could not initialize jenkins.model.Jenkins

Reference: http://bugs.java.com/bugdatabase/view_bug.do?bug_id=8051847
Reference: ZD-23093
Added a hook to perform additional diagnostic activities before killing the channel.

ssh-slaves plugin can leverage this to try to obtain the state of the slave JVM via SSH.

Reference: ZD-23093