Function map_gui::tools::prompt_to_download_missing_data

pub fn prompt_to_download_missing_data<A: AppLike + 'static>(
    ctx: &mut EventCtx<'_>, 
    map_name: MapName
) -> Transition<A>

Prompt to download a missing city. On either success or failure (maybe the player choosing to not download, maybe a network error), the new map isn't automatically loaded or anything; up to the caller to handle that.