Add a debugmsg when item_factory can't find an item.

I bumped into some "Error: Missing Item"s the other day. Unfortunately, since it doesn't say WHAT item was missing, this was extremely unhelpful, and so I have no idea what was wrong.

In the hopes of finding that out, and for helping anyone who mistypes a line in item_groups.json, I added an explicit debugmsg that contains the actual id of the item that was missing.