Resolve: better error message when resolve fails due to privateness

[kud1ing]

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"?

[jdm]

Nominating for production ready.