cmd/compile: better error message when offending/missing token is a keyword
Prefix keywords (type, default, case, etc.) with "keyword" in error
messages to make them less ambiguous.
Fixes #68589.
Change-Id: I1eb92d1382f621b934167b3a4c335045da26be9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/623819
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Tim King <taking@google.com>