Prevent the trade from crashing when an out-of-schema item is added

Fixes #390
When merged with #405, this will supersede #393.

Also fixes a copy paste error in #393 in the FireOnUserRemoveItem function.
Code style changes
Removes todos and prints to console
Merge pull request #413 from FunkyLoveCow/Branch_nullitem

Prevent the trade from crashing when an out-of-schema item is added