auto merge of #6443 : cmr/rust/resolution, r=bstrie

When trying to import nonexistent items from existing modules, specify that
that is what happened, rather than just reporting "unresolved name".

Ideally the error would be reported on the span of the import... but I do not see a way to get a span there. Help appreciated 😄

src/test/compile-fail/import2.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use baz::zed::bar;  //~ ERROR unresolved name
+use baz::zed::bar;  //~ ERROR unresolved import
 //~^ ERROR failed to resolve import
 
 mod baz {}