[B] Specify when a selected game mode is invalid. Fixes BUKKIT-5347

The Issue:  The current commands can provide confusion, as explained below.

Reasoning for PR: 
When the game mode command is run, and an invalid game mode is specified, it should state the error that was made, instead of defaulting to survival. 

The current command could display confusion as when a invalid game mode is specified, it would default to Survival and not what the user wanted,