Change Disconnect message to the Throwable's message

Throwable#toString() returns a description of the Throwable that
includes the name of the class appended to
Throwable#getLocalizedMessage(), which can be used for debugging
purposes.

src/main/java/org/spacehq/packetlib/tcp/TcpSession.java

@@ -352,7 +352,7 @@ public abstract class TcpSession extends SimpleChannelInboundHandler<Packet> imp
         } else if(cause instanceof WriteTimeoutException) {
             message = "Write timed out.";
         } else {
-            message = "Internal network exception.";
+            message = cause.toString();
         }
 
         this.disconnect(message, cause);