It might be useful to make this error message a little more verbose so the user understands a little clearer what is going on.
“Your game needs to be updated to a newer version to connect to this server. The server is running v1300, you are running v1297.”
“Your game cannot connect to this server, it’s owner needs to update it to a newer version before you can connect. The server is running v1301, you are running v1300.”
The mileage on the words will definitely vary and there is probably more to it but it would save the weekly confusion some players (usually new) are experiencing if it can be implemented by Facepunch easily.