compiler: improve error messages for expected curly brace

For golang/go#17328

Change-Id: Id0c153e76de2db5d15e768823d67646a4bde0e23
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/273890
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>