[Reactive] hint/warning when a candidate for generation is ignored for certain reason

[Xiaoy312]

example for a view model-class that properties of type IState/IFeed/IListFeed normally a bindable version of this vm would be generated by Uno.Extensions.Reactive.Generator however, if the said vm class doesnt have the partial modifier, the bindable vm is not generated

it would be nice to have some kind of build warning, or runtime log warning/error for this kind of problem

[nickrandolph]

@dr1rrb do we still need to add this?