Resolve: better error message when resolve fails due to privateness

Say you have a module `foo.rs` containing a `struct Foo;` where you've forgot to add a `pub`.

When you try to `use foo::Foo`, you'd get `error: failed to resolve import: foo::Foo`.
This may be puzzling.

How about a message in the tone of "there is a foo:Foo but that is private, so i am afraid i can't let you use it"?
