Tweak Out of Stock Message

[BFTrick]

When you have an item with 1 stock and you add it to your cart all it well. When you try to add it again you get an You already have this item in your cart. message which isn't the most intuitive. Can we change this to `You can't add any more of this item to your cart" or something like that?

ref: https://woothemes.zendesk.com/agent/#/tickets/103958