Handle invalid choices (e.g. trapping)

[taylorhansen]

Certain abilities keep the opponent from being able to switch, but is only partially revealed when the opponent tries to switch out.

Sample:

<< |request|{...(in RequestPokemon obj: maybeTrapped=true)}
>> /choose switch 2
<< |callback|trapped|0
<< |error|[Invalid choice] Can't switch: The active Pokémon is trapped

Parse the |callback| message and send it to the Battle instance so it can use that info to send a new Choice. The network won't need to know whether its choices are restricted, since it already ranks all of its available choices.