IGBR: mission failure penalty

[quick cedar]

In-Game Bug Report by Anonymous: fail a trade mission a few units short of the target and the units arent returned.

[tropic bloom]

it's at least fair to fail the trade when you're 2 units short, I hoped I would get a portion of the rewards but it's fine that I don't - but to have literally everything I've been working for the past 8 days to make just disappear is intensely frustrating

[frank fulcrum]

That's a really good point. Especially for some of the later trades which have huge item requirements. @granite scaffold Any thoughts on this?

This could potentially be a good fit for another world settings option to change the trade fail behavior.