Fanart/Poster image selection related cleanup

Simplify dlgImgSelect callers by returning a DialogResult from the ShowDialog overloads.
Expose the selected image via a public Results property. 

This way the local Poster/Fanart variable won't get set to Nothing while we scrape multiple movies and prevent a NULL-deref. 

Additionally:
* Don't display the selection dialog in cases where there is no image(s) to select while doing a scrape.
* Display a message box instead of an empty selection dialog when manually scrapin poster/fanart from the edit movie dialog.