librustc: Improve error message for missing feature attributes.
auto merge of #13471 : Ryman/rust/feature_syntax_error, r=brson

The current error message is misleading, it asks users to add `#[feature(..)]` which ends up being treated as an outer attribute, which then has no error unless `attribute_usage` lint is enforced. The code will still fail and the user might not understand why.