server: add log message when client connection is destroyed due to an error

The client connection is destroyed by the server in several
circumstances. This patch adds log messages in case the connection is
destroyed due to an error other than normal hangup.

Signed-off-by: Mathias Fiedler <mathias_fiedler@mentor.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>