Implement unreliable message size limit error.

Not a proper solution to the unreliable fragmentation issue, but better than nothing.

See lidgren#148

Lidgren.Network/NetUnreliableSenderChannel.cs

@@ -59,6 +59,17 @@ internal override NetSendResult Enqueue(NetOutgoingMessage message)
 				return NetSendResult.Dropped;
 			}
 
+			if (message.LengthBits >= ushort.MaxValue
+			    && m_connection.m_peerConfiguration.UnreliableSizeBehaviour == NetUnreliableSizeBehaviour.IgnoreMTU)
+			{
+			    // drop message
+			    this.m_connection.m_peer.LogError(
+			        string.Format("Unreliable message max size exceeded: {0} bits (max {1})",
+			                      message.LengthBits,
+			                      ushort.MaxValue));
+			    return NetSendResult.Dropped;
+			}
+
 			m_queuedSends.Enqueue(message);
 			m_connection.m_peer.m_needFlushSendQueue = true; // a race condition to this variable will simply result in a single superflous call to FlushSendQueue()
 			return NetSendResult.Sent;