Added type check and error message for unary minus on non-numeric types

On non-overloaded uses, of course.

I'm not quite sure on my use of res wholeAgain here but I can think of no other solution.
If I don't check against here, it could just be that an operator decl wasn't resolved and we got a -1 score when checking against it.
However, the code would still error out, although a valid overload could exist.

@nddrylliog, thoughts?