NetworkManager allows adding a game object with NetworkIdentity instead of prefab

1. What happened
User is able to add game object with NetworkIdentity instead of prefab, that causes errors and misleads user.
2. How we can reproduce it using the example you attached
To repro:
1. Create a new scene.
2. Create and empty object, add Network Manager and Network Hud components.
3. Create a cube and add NetworkIdentity component.
4. In NetworkManager - Spawn Info - Player Prefab add Cube.
5. Add scene to build settings and build.
6. In Player hit Host.
7. In Editor hit Play and Client.

Actual result:
1. In Host both object and player are spawned.
2. In Client only player is spawned and error in log:
Spawn scene object not found for 1
UnityEngine.Networking.NetworkIdentity:UNetStaticUpdate()

Expected result:
1. There should be no ability to add a non-prefab object to NetworkManager.
2. Error "Spawn scene object not found for 1" is not understandable. Error happens in OnObjectSpawnScene method, but error at least might point that digit is a scene ID.

Reproduced in 5.5.4f1, 5.6.1p2, 2017.1.0b7, 2017.1.0b8, 2017.2.0a2