go/types, types2: better error message for invalid ERROR pattern in tests
When unquoting of an ERROR or ERRORx pattern fails, say so instead of
simply printing "syntax error".
Change-Id: I586dffa86ca80f5b40a5cbe16a7005cc1f7862f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/586958
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com>