Better error for some unresolved imports

When trying to import nonexistent items from existing modules, specify that
that is what happened, rather than just reporting "unresolved name".
Use backticks to delineate paths/identifiers
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 :smile: