Wrong flow when user doesn't have enough ETH wrapped

[fvictorio]

Reproduction steps:

• With one account, sell 10 ZRX at 1 wETH each.
• With another account, wrap 4 ETH
• Buy 5 ZRX on market

Expected: You should have one step to wrap 1 ETH more, and then buy

Actual behavior: The buy transaction is sent and it fails.