Unhelpful error if a `Edges` struct contains two or more exclusive borrows of the same resource

[schell]

For example:

#[derive(CanFetch)]
struct SceneSetupData {
    entities: Write<Entities>,
    render_objects: RenderObjectData
}

will err during construction because RenderObjectData also has a Write<Entities>, but the error message is this:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: could not construct SceneSetupData

Caused by:
    'Write::construct' requested missing resource "apecs::world::Entities" encountered while building request: the borrow is exclusive and the resource is missing

[schell]

Maybe we can check the exclusivity of borrows at compile time in the derive macro?