Error documentation?

A lot of functions, e.g. http://docs.unity3d.com/ScriptReference/Networking.NetworkTransport.Connect.html, have an "out byte error", but I can't find any documentation on how to interpret that error byte.

What do the error codes mean? Is there documentation?

 

I think I figured it out, you cast the byte error to http://docs.unity3d.com/ScriptReference/Networking.NetworkError.html

But that raises the question -- why not simply have the transport layer return the type NetworkError?