refactor: Minor cleanups in Unity Transport

[simon-lemay-unity]

Just a few minor cleanups in Unity Transport:

• Remove the internal NetworkDriver accessor. There's a public GetNetworkDriver nowadays that can be used for the same purpose.
• Add a using directive for transport errors which avoids using the full (and long) type name.
• Remove the internal "state" member. The same information can be gleaned by looking at the driver's state and the value of m_ServerClientId. Plus, the Listening state did not have the same meaning as it does in NGO itself, which could lead to confusion.
• Simplify ErrorUtilities to avoid the useless constants and only providing strings for errors users can actually do something about. Also moved it to the bottom of the file. The top is too prime real estate for such a minor utility. :P

Changelog

N/A

Testing and Documentation

• No tests have been added.
• No documentation changes or additions were necessary.

[michalChrobot]

Looks good for me but I would wait for @NoelStephensUnity or @EmandM to approve. Do you think those changes should also be ported to develop branch (NGOv1.X)?

[simon-lemay-unity]

I don't think it's worth backporting those changes to version 1.X. They don't really fix anything and they're mostly cleanups I did while working on performance improvements and those will be 2.X-only.

[michalChrobot]

Okok, makes sense

[EmandM]

This looks much cleaner. Thank you!