SimpleUserHandler failsafes

Adds some failsafes: A try-catch for AcceptTrade() and a Poll on OnTradeReady()

It was discovered after testing that having a lot of items in the trade may cause AcceptTrade() to give an exception, making the bot crash even though the trade would go through.

The Trade.Poll is also added to OnTradeReady() to prevent another sort of crash that has been discussed recently.

It's best we add these in now rather than have more related issues from new users in the future.