Show bank: Only display "no-one has this trade" when the numbers vs the bank do not match
"It wasn't implemented with visible bank in order to save time, so people don't ask for a resource no one has and that everyone knows there is none in the board."
It should only do it if the numbers vs the bank do not match:
Like asking for a wood if there's 19 in the bank or asking for 2 wheat when there's 18 in the bank.
But if there's 18 wheat and 18 wood in the bank, player A has wood, player B has wheat, and you ask for 1 wood 1 wheat, you should not get the error.