Assert substitute type

[antongolub]

const decorator = constructDecorator(
          (targetType: unknown, target: any, age: number) => {
            if (targetType === CLASS) {
              return 'foo'
            }
            return
          },
        )

        @decorator(100)
        class Foo {
        }

should throw something like

new Error('Decorator: class decorator should return a class')

[antongolub]

This happens at runtime, so let the engine handle it.