go/types, types2: provide error codes where they were missing

go/types, types2: provide error codes where they were missing

We need a better approach eventually, but this is ok for now.
In go/types, always use _UnsupportedFeature for unavailable
version-specific features.

Change-Id: I15b47e34eda167db3133bd481aa2f55cf3662c31
Reviewed-on: https://go-review.googlesource.com/c/go/+/433195
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>