EXR + Use Z Buffer, without Z Output Connected

Display a little warning label on the Render Output panel when using Open EXR as file format, having Use Z Buffer on, and the Z output of the Composite node is not connected.

Not a lot of people might even stumble upon this, but is nice to warn when you explicitly have Use Z Buffer on, but you're about to get a completely and useless white 1.0 Z Buffer because you forgot to plug the Z output in the Composite node.