Don't write to connection if connection cannot be written to.

org.jivesoftware.openfire.nio.ConnectionHandler#exceptionCaught is triggered whenever an exception occurs with message processing. The standard response is to send an error to the peer, and close the connection.

if the exception that is caught relates to a problem sending messages to the peer, attempting to send an error (before closing) would likely cause further problems. We should avoid doing this.