Core: Add new error message for item count when defined as a set instead of a dict

[Zannick]

What is this fixing or adding?

If you, like me, forget that items have counts and attempt to give yourself a unique starting item by writing in your yaml:

    starting_items:
        {"Starting Item"}

You get an error TypeError: '<' not supported between instances of 'NoneType' and 'int'... because yaml interprets this as a dict with an implicit value of None and the original error message comes from checking whether any value is less than 1. This produces a better error message if any dict value is None.

How was this tested?

Manually.