Invalid room code feedback

[hobnob]

Is your feature request related to a problem? Please describe.
Entering a wrong room code just results in a blank room code

Describe the new feature you'd like
There's a disconnect between the server room codes and what the client can enter (for example a client can enter the letter i, which isn't valid). When this happens we don't feed back to the player at all and so they're left wondering what went wrong. At the earliest oppertunity we should send the code to the server for validation and reply back, letting the player know that something is wrong.